Rosetta is a deep space mission of
ESA , it is the first mission designed to both orbit and land on a
comet. ESA selected the mission in Nov. 1993 as the third cornerstone
mission in its long-term science program, called 'Horizon 2000'. The
goal of the Probe is to rendezvous with the Comet
67P/Churyumov-Gerasimenko and map its surface in fine detail. It will
also land a package of instruments (the Philae Lander) to study some of
the most primitive, unprocessed material in the solar system. 1)

Comets are among the most beautiful
and least understood nomads of the night sky. To date, half a dozen of
these most heavenly of heavenly bodies have been visited by spacecraft
in an attempt to unlock their secrets. All these missions have had one
thing in common: the high-speed flyby. Like two ships passing in the
night (or one ship and one icy dirtball), they screamed past each other
at hyper velocity — providing valuable insight, but fleeting
glimpses, into the life of a comet. That is, until Rosetta.

Launched in March 2004 and expected
to reach the comet by 2014, Rosetta will be the first mission to
revolve around the comet's nucleus and deliver a lander to its surface.
The spacecraft will be located at a distance of 600 million km or 4AU
(astronomical units) from the sun upon the comet.

• The Probe is named
after the famous Rosetta Stone, a slab of volcanic basalt found near
the Egyptian town of Rashid (Rosetta) in 1799 on the Nile island of
Philae (soldiers in Napoleon's army discovered the Rosetta Stone). An
obelisk found on Philae provided the French
scholar/historian/archeologist Jean-Francois Champollion (1790-1832)
with the final clues for deciphering the hieroglyphs on the Rosetta
Stone – thus the mission name. - The stone revolutionized our
understanding of the past. By comparing the three carved inscriptions
on the stone (written in two forms of Egyptian and Greek), historians
were able to decipher the mysterious hieroglyphics – the written
language of ancient Egypt. As a result of this breakthrough, scholars
were able to piece together the history of a lost culture. — The
Rosetta Stone provided the key to an ancient civilization. ESA’s
Rosetta mission will allow scientists to unlock the mysteries of the
oldest building blocks of our Solar System: comets.

• Rosetta's prime
objective is to help understand the origin and evolution of the Solar
System. The comet’s composition reflects the composition of the
pre-solar nebula out of which the Sun and the planets of the Solar
System formed, more than 4.6 billion years ago. Therefore, an in-depth
analysis of comet 67P/Churyumov-Gerasimenko by Rosetta and its lander
will provide essential information to understand how the Solar System
formed.

- There is convincing evidence
that comets played a key role in the evolution of the planets, because
cometary impacts are known to have been much more common in the early
Solar System than today. Comets, for example, probably brought much of
the water in today's oceans. They could even have provided the complex
organic molecules that may have played a crucial role in the evolution
of life on Earth.

• Ensuring that the
spacecraft survives the hazards of travelling through deep space for
more than ten years is therefore one of the great challenges of the
Rosetta mission.

• Spacecraft
hibernation: For much of the outward journey, the spacecraft will be
placed in 'hibernation' in order to limit consumption of power and
fuel, and to minimize operating costs. At such times, the spacecraft
spins once per minute while it faces the Sun, so that its solar panels
can receive as much sunlight as possible. - Almost all of the
electrical systems are switched off, with the exception of the radio
receivers, command decoders and power supply.

• The Comet
67P/Churyumov-Gerasimenko was discovered on September 9, 1969 –
by chance – by Klim Ivanovic Churyumov and Svetlana Ivanovna
Gerasimenko (of Kiev University) on photographic plates taken for comet
35P/Comas-Sola with the 50 cm Maksutov telescope of the Alma Ata
Observatory, Tadchik Republic.

Figure 1: The Rosetta Stone is displayed at the British Museum in London since 1802 (image credit: British Museum)

Legend to Figure 1:
The Rosetta Stone is a granodiorite stele inscribed with a decree
issued at Memphis in 196 BC on behalf of King Ptolemy V. The decree is
given in three languages: Egyptian hieroglyphs (top), Demotic (middle),
and ancient Greek (bottom). Champollion used the Greek to decipher the
hieroglyphs (a full decipherment was published in 1824). The Rosetta
Stone provided the key to the modern understanding of Egyptian
hieroglyphs. The Rosetta Stone has a size of 114.4 cm x 72.3 cm x 27.93
cm. 3)

- the determination of chemical, mineralogical and isotopic compositions of volatiles and refractories in the cometary nucleus

- the determination of the physical properties and interrelation of volatiles and refractories in the cometary nucleus

- studies of the development of
cometary activity and the processes in the surface layer of the nucleus
and inner coma, that is dust/gas interaction

- studies of the evolution of the interaction region of the solar wind and the outgassing comet during perihelion approach.

Spacecraft:

Rosetta is truly an international
enterprise, involving more than 50 industrial contractors from 14
European countries and the United States. The prime spacecraft
contractor is Airbus Defence and Space (formerly EADS Astrium, GmbH,
Friedrichshafen, Germany), responsible for building the spacecraft.
Other contributions were provided by Airbus Defence and Space UK
(spacecraft platform), and Airbus Defence and Space France (spacecraft
avionics) and Alenia Spazio (assembly, integration and verification)
are major subcontractors.

Rosetta comprises a large orbiter,
which was designed to operate for a decade, and a lander. Each of these
components carries a large array of scientific instruments that will
perform the most extensive study of a comet to date. The orbiter will
revolve around the comet from 1km distance examining the nucleus and
environment of the comet.

The Rosetta orbiter is a platform
of size 2.8 m x 2.1 m x 2 m (an aluminum box) with a payload support
module, which houses the 11 scientific instruments, mounted on the top
and a bus support module, housing the subsystems, on the base. Two sets
of solar panels with 14 m length having an area of 64m2
extend from the side of the spacecraft and a 2.2 m diameter steerable,
high-gain antenna dish sticks out from the front. The lander is
attached to the back of the orbiter. The two solar panel wings rotate
at ±180° to capture the maximum sunlight. 5)6)

The spacecraft is built around a
vertical thrust tube, whose diameter corresponds to the 1.194 m
Ariane-5 interface. This tube contains two large, equally sized
propellant tanks (each of 1106 liter), the upper one containing fuel,
and the lower one containing the (heavier) oxidizer. The Orbiter also
carries 24 thrusters for trajectory and attitude control. Each of these
thrusters pushes the spacecraft with a force of 10 Newton, equivalent
to that experienced by someone holding a bag of 10 apples. Over half
the launch mass of the entire spacecraft, about 1670 kg, is made up of
propellant.

The spacecraft is three-axis
stabilized. Attitude is maintained by four reaction wheels as well as
using two star trackers, sun sensors, navigation cameras, and three
laser gyro packages. Power is supplied by the solar arrays. The solar
cells employed are 200 µm Si solar cells of LLIT (Low Intensity,
Low Temperature) type sized 37.75 mm in width and 61.95 mm in length
(Figure 3). The cover glass is 100
µm thick ceria doped micro-sheet designated curb mount glass
(CMG). The cover-glass covers the solar cell completely. The solar
arrays will provide 395 W at 5.25 AU and 850 W at 3.4 AU, when comet
operations begin. Power is stored in four 10 Ah NiCd batteries which
supply the 28 V bus power.

Figure 3: Close-up of a single solar array cell (image credit: ESA)

The Rosetta deep space mission of ESA represents a rather special case of solar cell technology use.
At its destination in 2014, the spacecraft is at a distance of about
675 million km from Earth, corresponding to 4.5 AU, a distance almost
as far out as Jupiter , where sunlight levels are only 4% of those on
Earth. In LEO (Low Earth Orbit), Rosetta is the most powerful
spacecraft that ESA ever built, at 12 kW of installed solar power
generation provided by two solar arrays, covered with hundreds of
thousands of specially developed non-reflective silicon cells. But at
the deep space distance of Comet 67P/Churyumov-Gerasimenko, the total
solar power available is only ~400 W. - Rosetta is the first deep
space mission ever to rely entirely on solar power generation beyond
the main asteroid belt (with sunlight levels of only 3-4% as those in
LEO) - without the use of RTG (Radioisotope Thermoelectric Generator)
technology (as is being done by all other deep space satellites).

Illumination reduces with distance
from the Sun by what is called the inverse square law – if one
goes twice as far away only a quarter the solar intensity is available,
at three times the distance, only one ninth of the intensity is
available. This means that temperatures experienced by the spacecraft
also fall with distance, though in principle this is good news for
solar cell designers, since cell efficiency increases as the
temperature goes down.7)8)9)10)

In practice however, in low solar
intensities with temperatures dropping below –100°C, standard
solar arrays show worse-than-expected performance due to unpredictable
degradation of individual cells. To overcome this problem, the LLIT
(Low Intensity Low Temperature) specific solar cell technology was
developed. The resulting single-junction silicon cells are flying on
ESA’s Rosetta comet chaser, which is venturing three times
further from the Sun than Earth.

Figure 4:
Photo of one of Rosetta’s two massive solar wings (each with 5
solar panels), keeping Rosetta powered out in the cold depths of space
(image credit: ESA)

Legend to Figure 4:
The image was taken in 2002 showing Rosetta being checked in ESA/ESTEC
in Noordwijk, The Netherlands. One hinged wing is supported on a rig to
allow it to unfurl safely in Earth gravity instead of weightlessness.
Each wing is made up of five hinged panels, the steerable pair of wings
together stretches 32 m tip-to-tip from the box-shaped spacecraft. 11)

At Rosetta's encounter with the
Comet, the spacecraft is experiencing only 11% of Earth-level solar
illumination — but still better than the 4% when it was furthest
from the Sun. But instead of going nuclear, Rosetta runs solely on LILT
(Low-Intensity, Low-Temperature) silicon solar cells, a new European
technology devised for this mission, optimized for deep-space
conditions. - The same is true of Rosetta’s Philae lander, whose
batteries are designed to be recharged by the LILT cells covering its
body.

RF communications:
Communications is maintained via the high-gain antenna, a fixed 0.8 m
medium-gain antenna, and two omnidirectional low gain antennas. Rosetta
utilizes an S-band telecommand uplink and S- and X-band telemetry and
science-data downlinks, with data transmission rates from 5 to 20
kbit/s. The communication equipment includes a 28 W RF X-band TWTA
(Traveling Wave Tube Amplifier) and a dual 5 WRF S/X band transponder.
Onboard heaters keep the instrumentation from freezing during the
period the spacecraft is far from the sun.

The Rosetta spacecraft had to be
designed for a high level of reliability as the main scientific mission
is starting more than 10 years after launch, and also for a high level
of availability during the early years of the mission cruise, which
also contains many key technical and scientifically valuable events
including three Earth swingbys, one Mars swingby, two asteroid flybys,
several deep space trajectory correction maneuvers, and regularly
scheduled onboard system and payload checkouts.

These outstanding efforts will
assure that Rosetta will contribute significantly to answer open
questions in solar system research such as: How pristine are comets?
How does cometary activity work? Are the craters on comets from impacts
or from other processes? What is the internal structure of cometary
nuclei? How does cometary material look like and what is it made of?
Are there internal heat sources that trigger normal activity and
outbursts? What are the main physical and chemical processes in the
coma? How does solar wind – comet interaction change at the
different activity levels from 3 AU to perihelion? Are comets
candidates that delivered prebiotic molecules and water to Earth?

Comets remain the poorest
understood solar system objects. The future measurements of Rosetta
orbiting around a comet for several months and delivering a lander to
the surface will open a whole new field of research. And Rosetta will
provide a much better understanding of comets and solar system
formation, much as the Rosetta Stone did in our understanding of the
Egyptian culture (Ref. 4).

NAVCAM (Navigation Camera):
The Rosetta spacecraft uses a single camera with a 5º field of
view and 12 bit 1024 x 1024 pixel resolution, allowing for visual
tracking on each of the spacecraft approaches to the asteroids and
finally to the comet.

Thermal louvers: Throughout
its mission, the Rosetta spacecraft is exposed to extreme cold and hot
temperatures. In the early and late stages of its prolonged expedition,
the spacecraft will sweep across the inner Solar System, where sunlight
is plentiful. However, in order to rendezvous with Comet
67P/Churyumov-Gerasimenko, Rosetta will have to probe beyond the
asteroid belt, more than 5 times the Earth's distance from the Sun. In
those frigid regions, the solar energy levels are only 4% of the those
that we enjoy on our balmy planet. 12)

Since it is not feasible to wrap a
spacecraft in multiple layers of warm clothing for periods of deep
freeze, then strip these away when sunbathing is the order of the day,
the ESA team has been obliged to come up with alternative ways of
regulating temperature.

Designers have provided Rosetta
with louvers — high-tech venetian blinds which control the
spacecraft's heat loss. Lovingly polished by hand, these assemblies of
thin metal blades must be handled like precious antiques, since any
scratching, contamination or fingerprints will degrade their heat
reflecting qualities.

The principle behind the louvers is
quite simple. When Rosetta is cruising around the inner Solar System
and basking in the warmth of the Sun, surface temperatures may soar to
130°C, and even internal equipment can reach 50°C. At such
times, it is vital to stop the spacecraft from overheating, so the
louvers are left fully open, allowing as much heat as possible to
escape into space from Rosetta's radiators.

However, during its prolonged deep
space exploration and comet rendezvous phases, when temperatures
plummet to -150°C, heat conservation is the order of the day. Since
the spacecraft's limited internal power supply - equivalent to the
output from three ordinary light bulbs - then becomes the main source
of warmth, it is essential to trap as much heat as possible. This means
completely closing the louvers in order to prevent any heat from
escaping.

Some 14 of these louver panels cover an area of 2.25 m2
on the Rosetta spacecraft, placed over its radiators across the side
and back of the spacecraft. The louvers open and close on a fully
passive basis, requiring no power to operate. Instead they work on a
‘bimetallic’ thermostat principle. The blades are moved by
coiled springs made up in this case of a trio of different metals that
expand and contract at differing rates, precisely tailored to rotate as
required. Designed by Spain’s Sener company, the louvers were
extensively tested by ESA’s Mechanical Systems Laboratory in
advance of Rosetta’s 2004 launch. 13)

Figure 6: Photo of a louver panel (image credit: ESA–A. Le Floc'h)

Figure 7:
Photo of the Rosetta spacecraft with thermal blankets, released on
January 19, 2004, ready for testing in the Large Space Simulator, at
ESA/ESTEC (image credit: ESA) 14)

Legend to Figure 7:
Temperature control was a major headache for the designers of the
Rosetta spacecraft. Near the Sun, overheating has to be prevented by
using radiators to dissipate surplus heat into space. In the outer
Solar System, the hardware and scientific instruments must be kept warm
(especially when in hibernation) to ensure their survival.

Figure 8: Rosetta and Philae
— the lander is attached to the orbiter during integration of the
spacecraft (image credit: ESA)

Legend to Figure 9:
Rosetta is the first spacecraft to journey beyond the main asteroid
belt and rely solely on solar cells for power generation. The new
solar-cell technology used on its two giant solar panels allow it to
operate over 800 million km from the Sun, where light levels are only
4% of those on Earth.

Launch: The Rosetta spacecraft was launched on March 2, 2004 on Ariane 5G+ vehicle from Kourou, French Guinea.

Orbit and mission event overview:

The original target of the Rosetta
mission was comet 46P/Wirtanen. A failure of an Ariane-5 rocket in
December 2002 forced ESA to postpone the initially scheduled January
2003 launch and to re-target Rosetta, now heading for Comet
67P/Churyumov-Gerasimenko (Ref. 4).

Rosetta could not head straight for
the comet. Instead it began a series of looping orbits around the Sun
that brought it back for three Earth fly-bys and one Mars fly-by. Each
time, the spacecraft changed its velocity and trajectory as it
extracted energy from the gravitational field of Earth or Mars. During
these planetary fly-bys, the science teams checked out their
instruments and, in some cases, took the opportunity to carry out
science observations coordinated with other ESA spacecraft such as Mars
Express, Envisat and Cluster. 15)

During the 10 year trek across our
solar system, Rosetta will travel five times the distance Sun-Earth,
and will pass through the asteroid belt into deep space beyond 5 AU
solar distance before it reaches its destination, the periodic comet
67P/Churyumov-Gerasimenko. On its way to 67P/Churyumov-Gerasimenko the
spacecraft will employ four planetary gravity assist maneuvers
(Earth-Mars-Earth-Earth) to acquire sufficient energy to reach the
comet (Figure 10). Each of the fly-bys
required months of intense preparation. In particular the fly-by of
Mars in February 2007 was a critical operation: the new mission
trajectory to 67P/Churyumov-Gerasimenko required that Rosetta fly past
Mars at just 250 km from the surface, and spend 24 minutes in its
shadow.

In between the last two Earth
swingbys Rosetta will fly by the main belt asteroid 2867 Steins at a
distance of 1700 km and at a relative velocity of 9 km/s on September
5, 2008. After the third Earth swingby Rosetta will enter the main
asteroid belt again and fly by the main belt asteroid 21 Lutetia at a
distance of 3000 km and a speed of 15 km/s on July 10, 2010. The
spacecraft will enter a hibernation phase in July of 2011. In January
2014 Rosetta will come out of hibernation and begin a series of
rendezvous maneuvers for comet 67P/Churyumov-Gerasimenko in May 2014.

The rendezvous maneuver 2 at~4.5 AU
from the Sun will lower the spacecraft velocity relative to that of the
comet to about 25 m/s and put it into the near comet drift phase,
starting May 22, 2014 until the distance is about 10,000 km from the
comet (Figure 3). It will be performed on the basis of a ground-based
determination of the orbit from dedicated astrometric observations,
before the comet is detected by the on-board cameras. The final point
of the near-comet drift phase, the CAP (Comet Acquisition Point), is
reached at a Sun distance of less than 4 AU. As soon as the spacecraft
with a maximum relative velocity of about 1 m/s. The time and direction
of the Rosetta-Philae separation will be chosen such that the landing
package arrives with minimum vertical and horizontal velocities
relative to the local (rotating) surface. After delivery of the lander
on November 10, 2014 at a solar distance of 3 AU, the spacecraft will
be injected into an orbit which is optimized for receiving the data
transmitted from the lander and to relay them to the Earth. To adjust
the payload operations sequences, the lander can be commanded via the
orbiter.

The Orbiter’s scientific
sensor complement includes 11 experiments and a small Lander, which
will conduct its own scientific investigations. Scientific consortia
from institutes across Europe and the United States have provided these
state-of-the-art instruments.

The instruments on the Rosetta
Orbiter will examine every aspect of the small cosmic iceberg. Wide and
Narrow Angle Cameras will image the comet’s nucleus to determine
their volume, shape, bulk density and surface properties. Three
spectrometers operating at different wavelengths will analyze the gases
in the near-nucleus region, measure the comet’s production rates
of water and carbon monoxide/dioxide, and map the temperature and
composition of the nucleus (Ref. 4).

Our knowledge of the nucleus should
be revolutionized by the CONSERT experiment, which will probe the
comet’s interior by transmitting and receiving radio waves that
are reflected and scattered as they pass through the nucleus. 16)

Four more instruments will examine
the comet’s dust and gas environment, measuring the composition
and physical characteristics of the particles, e.g. population, size,
mass, shape and velocity. The comet’s plasma environment and
interaction with the electrically charged particles of the solar wind
will be studied by the Rosetta Plasma Consortium and the Radio Science
Investigation.

The objective of the OSIRIS
assembly is to observe the cometary rotation, and to study the physical
and chemical processes that occur in, on, and near the cometary
nucleus. It also maps the cometary morphology, which will help
Rosetta’s lander (Philae) to find a suitable spot for setting
down in the comet’s surface. -The strength of OSIRIS is the
coverage of the whole nucleus and its immediate environment with
excellent spatial and temporal resolution and the spectral sensitivity
across the whole reflected solar continuum up to the onset of thermal
emission. This provides a context for the interpretation of the results
from Philae. 18)19)20)

The OSIRIS cameras were provided by
a consortium of 9 institutes from 5 European countries and from ESA,
under the leadership of the MPS . The participating institutes of the
consortium are: MPS (Katlenburg-Lindau, Germany), LAM (Laboratoire
d’Astrophysique de Marseille), Marseille, France; UPD (University
of Padova), Padova, Italy; IAA (Instituto de Astrofísica de
Andalucía ), Granada, Spain; University of Uppsala (Sweden);
ESA/ESTEC, Noordwijk, The Netherlands; UPM (Universidad
Politécnica de Madrid), Madrid, Spain; INTA (National Institute
for Aerospace Technology),Madrid, Spain; IDA (Institute of Computer and
Network Engineering at the TU Braunschweig),Braunschweig, Germany.

Figure 13: Photo of the OSIRIS cameras (image credit: MPS)

After the launch of Rosetta (March
2, 2004), OSIRIS was activated on several occasions before the arrival
to the main target, comet 67P/Churyumov-Gerasimenko. It was
commissioned in seven slots between March 2004 and June 2005, and it
performed several important scientific observations:

• A monitoring campaign of comet 9P/Tempel 1 around the Deep Impact event on 4 July 2005

• The fly-by of asteroid 2867 Steins on 5 September 2008

• Two Earth swing-bys in Nov. 2007 and Nov. 2009

• The observation of the
remnant of a collision between two main-belt asteroids in February 2010
- The flyby of asteroid 21 Lutetia on 10 July 2010

• Early observation of the comet from more than 1AU distance in March 2011.

Parameter

NAC (Narrow Angle Camera)

WAC (Wide Angle Camera)

Optical design

3-mirror off-axis, equipped
with two filter wheels containing 8 positions each; a flat-field three
anastigmatic mirror system is adopted

2-mirror off-axis, equipped with two filter wheels containing 8 positions each; a two aspherical mirror system is adopted

Angular resolution (IFOV)

18.6 µrad/pixel (3.8 arcsec)

101 µrad/pixel (20.5 arcsec)

Focal length

717.4 mm

140 mm (sag), 131 mm (tan)

F-number

8

5.6

FOV

2.2º x 2.2º

12º x 12º

Spatial scale from 100 km

1.86 m/pixel

10.1 m/pixel

Typical filter bandpass

40 nm

5 nm

Wavelength range

250 - 1000 nm

240 - 720 nm

Number of filters

12

14

CCD detectors

2048 x 2048 (backside illuminated)

2048 x 2048 (backside illuminated)

Pixel size

13.5 µm

13.5 µm

Mass of camera

13.2 kg

9.48 kg

Figure 14: Specification of the OSIRIS assembly

Science objectives: The OSIRIS
science objectives for the comet nucleus, the gases and dust produced
by the comet and for the asteroid flybys are:

Objective

Method

Initial detection of nucleus

Detection of motion of nucleus against background stars from > 1 Mkm with multiple NAC images.

Detailed determination of size, shape, and volume to sufficient accuracy to constrain the density.

In orbit images with both cameras from < 100 km followed by shape deconstruction on ground.

Search for residual evidence of formation mechanisms and scale lengths.

High resolution, color imaging of the surface.

Investigation of topographic features and associated physical processes.

High resolution, color imaging of specific surface features and outgassing.

Mapping the surface variegation.

Global mapping at better than 1 m resolution.

Investigate the color and mineralogy of the surface to study the degree of inhomogeneity.

Global mapping in specific mineralogical bands.

Determine the mass loss rate.

Measurement of the depth eroded by activity with a resolution of 0.2 m or better.

Determine the effect of non-gravitational forces on the nucleus.

Repeated determination of the angular momentum vector and the instantaneous spin axis through perihelion.

Characterize the Philae landing site.

High resolution imaging of the target site.

Analyze short-term variability and outbursts.

Rapid imaging of active regions.

Figure 15: Nucleus objectives of OSIRIS

Objective

Method

Search for evidence of crustal diffusion.

High signal to noise ratio imaging of weak emission.

Search for gravitationally-bound material.

High resolution imaging and tracking of bright large dust particles in the coma; Stereo measurements using both cameras.

Search for evidence of particle fragmentation, acceleration, condensation, and optical effects close to the dust source.

High resolution imaging of the dust emission immediately above the source.

Determine the near-surface flow-field of dust and its temporal evolution.

Mapping of the dust distribution around the nucleus with a wide field of view.

Determine the optical and physical properties of the dust and estimate the dust size distribution.

Multi-phase angle and multi-color imaging.

Investigate night-side activity.

High signal to noise, low straylight measurements of the night side limb.

Quantify thermal inertia effects on emission.

Monitoring of active regions as the solar zenith angle varies.

Table 4: Dust objectives of OSIRIS

Objective

Method

Investigate the chemical inhomogeneity of active region.

Multi-wavelength studies of individual active regions.

Investigate the changes in volatile emission with heliocentric distances.

Monitoring of an active region from high heliocentric distance through to perihelion.

Identify scale lengths for dissociation of water molecules.

Measure cometocentric distance dependence of OI and OH emission.

Determine the onset of emission.

High signal to noise measurements of dust and CN emission.

Investigate the relationship between the dust distribution and the gas distribution in the coma.

Multi-wavelength studies of different species and comparison with the dust distribution using the wide angle camera.

Investigate the distribution of alkali metals in active regions and on emitted dust grains.

Studies of Na and its relationship to the dust distribution.

To study the nitrogen and sulphur chemistry in the nucleus.

Monitoring of CS, NH and NH2 emission.

Table 5: Gas objectives of OSIRIS

Objective

Method

Determine the sizes, volumes, and densities of the asteroids.

Resolved imaging over the rotation periods of the targets.

Derive surface reflectance properties and hence acquire information on the properties of the regolith.

Multiple phase angle observations and absolute calibrated data.

Study their surface morphologies and estimate their surface ages.

High resolution imaging of surface features and crater statistic measurements.

Study the mineralogical composition and its homogeneity.

Multi-filter high resolution imaging covering the NIR bands of olivine and pyroxene in detail.

Search for potential asteroid satellites.

Wide-angle coverage around closest approach.

Search for evidence of water.

Measurement of the water of hydration feature at 700 nm.

Table 6: Asteroid flyby objectives of OSIRIS

Objective

Method

To study the global meteorological conditions on Mars over a two-day period.

Multi-wavelength studies of the disc.

Investigate the vertical structure of aerosols in the Martian atmosphere.

Multi-wavelength resolved images of the limb.

Investigate the global chemical heterogeneity on Mars.

Multi-filter images of the surface using the narrow angle camera (concentrating on the near-IR from 650 to 1000 nm)

Investigate the global chemical heterogeneity on Phobos and Deimos.

High signal to noise resolved multi-wavelength images of the two satellites.

Search for evidence of the dissociation products of water.

OH and OI measurements.

Table 7: Mars and Martian satellites flyby objectives of OSIRIS

Objective

Method

Study the distribution of atomic oxygen emission in the upper atmosphere of the Earth.

Global OI imaging with the wide angle camera.

Investigate the global chemical heterogeneity on the Moon.

Multi-wavelength images of the surface using the NAC (concentrating on the near-IR from 650 to 1000 nm).

Search for evidence of surface sputtering and outgassing from the Moon.

Images of the Na distribution.

Perform calibration of the imaging system.

Multi-wavelength, multi-phase angle coverage of the Moon.

Table 8: Earth/Moon system flyby objectives of OSIRIS

ALICE (Ultraviolet Imaging Spectrometer)

Analyses gases in the coma and tail
and measures the comet’s production rates of water and carbon
monoxide/dioxide. ALICE is a NASA instrument providing UV spectroscopy
in the band 70-205 nm. PI: Alan Stern, SwRI (Southwest Research
Institute), Boulder, CO, USA. The ALICE UV spectrometer will analyze
gases in the coma and the tail, and it measures the comet’s
production rates of water and carbon monoxide or dioxide. It will also
provide information on the surface composition of the nucleus. 21)22)

Instrument:
Light enters the Alice telescope through a 40 x 40 mm entrance aperture
and is collected and focused by an off-axis paraboloidal primary mirror
onto the approximately 0.1° x 6° spectrograph entrance slit.
After passing through the entrance slit, the light falls onto the
toroidal holographic grating of a Rowland Circle style imaging
spectrograph, where it is dispersed onto a microchannel plate detector.
The 2D (1024 x 32 pixel) format MCP detector uses dual, side-by-side,
solar-blind photocathodes of potassium bromide (KBr) and cesium iodide
(CsI). The predicted spectral resolving power
(λ/Δλ) of Alice is in the range of 105 - 330 for an
extended source that fills the instantaneous FOV (Field of View)
defined by the size of the entrance slit. 23)

An imaging spectrometer that
combines three data channels in one instrument. Two of the data
channels are designed to perform spectral mapping. The third channel is
devoted to spectroscopy. Maps and studies the nature of the solids and
the temperature on the surface of the nucleus. Also identifies comet
gases, characterizes the physical conditions of the coma and helps to
identify the best landing sites. Provides VIS and IR mapping
(spectroscopy) in the region 0.25-5 µm. PI: Angioletta Coradini,
IAS-CNR, Rome, Italy. 25)

Instrument: The optical subsystems
are housed inside a common structure - the cold box - cooled to 130 K
by a radiative surface supported on a truss having low thermal
conductivity. On the pallet supporting the truss, two sets of
electronics and two cryogenic coolers for the detectors are mounted.
The cold box is rigidly mounted on the pallet but thermally isolated
from it. The pallet and cold box together form the optics module, which
is mounted inside the spacecraft arranged so that the observing axes of
the optical subsystems are normal to the nadir (comet) pointing wall of
the spacecraft. The electronics module, containing the digital
electronics and power supply, is mounted separately. 26)

The mapping channel optical system
is a Shafer telescope matched through a slit to an Offner grating
spectrometer. The Shafer telescope consists of five aluminum mirrors
mounted on an aluminum optical bench. The primary mirror is a scanning
mirror driven by a torque motor. The Offner spectrometer consists of a
relay mirror and a spherical convex diffraction grating, both made of
glass.

The mapping
channel utilizes a silicon charge coupled device (CCD) to detect
wavelengths from 0.25 µm to 1 µm and a mercury cadmium
telluride (HgCdTe) infrared focal plane array (IRFPA) to detect from
0.95 µm to 5 µm. The IRFPA is cooled to 70 K by a Stirling
cycle cooler. The cold tip of the cooler is connected to the IRFPA by
copper thermal straps. The CCD is operated at 155 K and is mounted
directly on the spectrometer.

The high resolution channel is an
echelle spectrometer. The incident light is collected by an off-axis
parabolic mirror and then collimated by another off-axis parabola
before entering a cross-dispersion prism. After exiting the prism, the
light is diffracted by a flat reflection grating, which disperses the
light in a direction perpendicular to the prism dispersion. The low
groove density grating is the echelle element of the spectrometer and
achieves very high spectral resolution by separating orders seven
through sixteen across a two-dimensional detector array.

The high-resolution channel employs
a HgCdTe IRFPA to perform detection from 2 to 5 μm. The detector is
cooled to 70 K by a Stirling cycle cooler.

Instrument

Mapping Spectrometer

High Resolution Spectrometer

Visible Channel

Infrared Channel

Infrared Channel

Spectral range

0.25 - 1.0 µm

0.95 - 5 µm

2.03 - 5.03 µm

Spectral resolution

100 - 380 λ/Δλ

70 - 360 λ/Δλ

1300 - 3000 λ/Δλ

FOV (mrad × mrad)

64 (slit) x 64 (scan)

64 (slit) x 64 (scan)

0.583 x 1.749

Instrument mass

30 kg

Table 10: Summary of VIRTIS characteristics

Status:

• Aug. 1, 2014: ESA’s
Rosetta spacecraft has made its first temperature measurements of its
target comet, finding that it is too hot to be covered in ice and must
instead have a dark, dusty crust. The observations were made by the
VIRTIS instrument between 13 and 21 July, when Rosetta closed in from
14 000 km to the comet to just over 5000 km. 27)

• On 14 July, 2014, the entire
surface of the comet occupied one of VIRTIS's pixels, allowing the
scientists to estimate the mean temperature of the nucleus –
around 205 K. While this may seem rather cold, it is somehow warmer
than the scientists expected, providing the scientists with some first
clues on the composition and the physical properties of the surface of
the nucleus.

• July 8, 2014: First
measurements of VIRTIS on board Rosetta have been probing the surface
temperature on the nucleus of comet 67P/C-G. 28)

MIRO (Microwave Instrument for the Rosetta Orbiter)

The MIRO investigation addresses
the nature of the cometary nucleus, outgassing from the nucleus and
development of the coma as strongly interrelated aspects of cometary
physics. During the flybys of the asteroids and, the MIRO instrument
will measure the near surface temperature of these asteroids and search
for outgassing activity in an effort to understand better the
relationship between comets and asteroids.

PI: Sam Gulkis of NASA/JPL,
Pasadena, CA. The MIRO investigation was conceived and designed
functionally by the investigation team consisting of 19 scientists from
6 different institutions, and the MIRO project office, located at
NASA/JPL.

1) Characterize the abundances of major volatile species and key isotope ratios in the nucleus ices.

The MIRO instrument will measure absolute abundances of key volatile species - H2O, CO, CH3OH, and NH3 - and quantify fundamental isotope ratios - 17O,16O and 18O, 16O - in a region within several kilometers from the surface of the nucleus, nearly independent of orbiter to nucleus distance.

Water and carbon monoxide are
chosen for observation because they are believed to be the primary ices
driving cometary activity. Methanol is a common organic molecule,
chosen because it is a convenient probe of gas excitation temperature
by virtue of its many transitions. Knowledge of ammonia abundance has
important implications for the excitation state of nitrogen in the
solar nebula. By providing measurements of isotopic species abundances
with extremely high mass discrimination, the MIRO experiment can use
isotope ratios as a discriminator of cometary origins. The MIRO
investigation will combine measurements of the variation of outgassing
rates with heliocentric distance with models of gas votalization and
transport in the nucleus to quantify the intrinsic abundances of
volatiles within the nucleus.

2) Study the processes controlling outgassing in the surface layer of the nucleus.

The MIRO experiment will measure surface outgassing rates for H2O,
CO, and other volatile species, as well as nucleus subsurface
temperatures to study key processes controlling the outgassing of the
comet nucleus.

The MIRO experiment will measure
surface outgassing rates for H2O, CO, and other volatile species, as
well as nucleus subsurface temperatures to study key processes
controlling the outgassing of the comet nucleus.

3) Study the processes controlling
the development of the inner coma. -MIRO will measure density,
temperature, and kinematic velocity in the transition region close to
the surface of the nucleus.

Measurements of gas density,
temperature, and flow field in the coma near the surface of the nucleus
will be used to test models of the important radiative and dynamical
processes in the inner coma, and thus improve our understanding of the
causes of observed gas and dust structures. The high spectral
resolution and sensitivity will provide a unique capability to observe
Doppler-broadened spectral lines at very low temperatures.

4) Globally characterize the nucleus subsurface to depths of a few centimeters or more.

The MIRO instrument will map the
nucleus and determine the subsurface temperature distribution to depths
of several centimeters or more. Morphological features on scales as
small as 5 m will be identified and correlated with regions of
outgassing.

The combination of global
outgassing and temperature observations from MIRO and in situ
measurements from the Rosetta lander will provide important insights
into the origins of outgassing regions and of the thermal inertia of
subsurface materials in the nucleus.

5) Search for low levels of gas in the asteroid environment.

The MIRO
instrument will search for low levels of gas in the vicinity of
asteroids and measure subsurface temperature to provide information on
the presence of water ice, and on near surface thermal characteristics
and the presence or absence of a regolith.

MIRO is configured both as a
continuum radiometer and a very high spectral resolution line receiver.
Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz
(0.5 mm). The spatial resolution of the instrument, operating in the
submillimeter band, is approximately 5 m at a distance of 2 km from the
nucleus. The MIRO spectrometer is tuned to measure four volatile
species - H2O, CO, CH3OH, and NH3 and the isotopes of water —H217O and H218O.
These four species have all been measured to be present in comets. The
spectral resolution is sufficient to observe individual, thermally
broadened, line shapes at all temperatures down to 10 K or less. The
MIRO experiment will use these species as probes of the physical
conditions within the nucleus and coma. The basic quantities measured
by MIRO are surface temperature, and gas abundance, velocity, and
temperature of each species, along with their spatial and temporal
variability. This information will be used to infer coma structure and
outgassing processes, including the nature of the nucleus/coma
interface. 30)31)

Instrument: MIRO consists of an assembly of two heterodyne radiometers:

- Millimeterwave receiver (at 190 GHz , ~ 1.6 mm)

- Submillimeterwave receiver (at 562 GHz, 0.5 mm).

The mm and sub-mm radiometers are
configured with a broadband continuum detector for the determination of
the brightened temperature of the comet nucleus and the target
asteroids. The sub-mm receiver is configured as a very high resolution
spectrometer for the observation of the eight molecular transitions.
The instrument is constituted of four separate physical modules,
interconnected by a harness. The sensor unit is mounted on the
spacecraft payload plane using the baseplate as the interface. The
telescope boresight direction is aligned with the Rosetta payload line
of sight. The optical bench is mounted on the undersite of the
baseplate, under the telescope and inside the spacecraft. The mm and
sub-mm wave receiver front ends, the calibration mechanism and the
quasi-optics for coupling the telescope to the receivers are installed
on the optical bench. The Sensor Backend Electronic Unit contains the
intermediate frequency processor, the PLL (Phase Locked Loop) and the
frequency sources. Due to his power consumption, it mounted next to a
louvred radiator internal to the spacecraft. The Electronic Unit
contains the CTS (Chirp Transform Spectrometer), including instrument
computer and power conditioning circuits. The USO (Ultra Stable
Oscillator) is self contained and thermally controlled. Those items are
presented in Figure 17.

MIRO post launch results: The MIRO
instrument appears to be fully functional at the end of commissioning.
The measured key parameters appear to be better than expected.

Parameter

As designed

As measured in-orbit

Millimeter (mm) beam width

25 arcmin

23.8±1.2 arcmin

Sub-mm beam width

10 arcmin

7.5 ± 0.25 arcmin

Sensitivity(mm continuum)

1 K in 1 sec

0.1 K in 1 sec

Sensitivity (sub-mm continuum)

1 K in 1 sec

0.3 K in 1 sec

Sensitivity (sub-mm spectroscopic

2 K in 2 min (300 kHz (dsb)

< 1 K in 2 min (300 kHz dsb)

Spectral resolution

50 kHz (0.027 km/s)

To be measured

Table 12: Key parameters were measured using the Earth as target, they are summarized in this table

ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis)

Comets are believed to be the most
pristine bodies in the solar system. They were created 4600 million
years ago far away from the sun and have remained for most of the time
of their existence far outside of Pluto’s orbit. They are small
enough to have experienced almost no internal heating. They therefore
present a reservoir of well-preserved material from the time of the
Solar System’s creation. They offer clues to the origin of the
Solar System’s material and to the processes that led from the
solar nebula to the formation of planets. In contrast to meteorites
(the other primitive material available for investigations), comets
have retained the volatile part of the solar nebula. Several
interesting questions on the history of the Solar System materials can
therefore be answered only by studying comets. In particular, the
composition of the volatile material — the main goal of the
ROSINA instrument.

ROSINA is the main mass
spectrometer on the orbiter of ESA’s Rosetta mission to comet
67P/Churyumov-Gerasimenko. It consists of two mass spectrometers for
neutrals and ions. ROSINA’s primary objective is to determine the
basic properties of the gas in the comet’s atmosphere and
ionosphere such as composition, temperature and velocity. 32)33)34)35)

Science objectives: ROSINA’s
main goal is to determine the elemental, isotopic, and molecular
composition of the comet’s atmosphere and ionosphere. In
addition, the scientists are interested in the temperature and the bulk
velocity of the gas as well as the reactions of the gas and ions with
the dust emitted by the comet. These results may render important
implications for questions regarding the origin of comets, the relation
between cometary and interstellar material, and the origin and
evolution of the Solar System.

To accomplish these demanding objectives, ROSINA has unprecedented capabilities:

• A wide mass range from 1 amu
(atomic mass unit) to more than 300 amu. This makes it possible to
detect light atoms such as hydrogen as well as heavy organic molecules.

• A high mass resolution of more than 3000 m/Δm. This means that ROSINA is, for example, able to resolve CO from N2 and 13C from 12CH.

• A wide dynamic range of 1010

• High sensitivity (more than 10-5
A/mbar) to accommodate large differences in ion and neutral gas
concentrations and severe changes in the ion and gas flux as the
comet’s activity develops between aphelion and perihelion.

Instrument: ROSINA consists
of two mass spectrometers for neutrals and primary ions with
complementary capabilities and a pressure sensor. The total mass of the
ROSINA assembly is 36 kg with a power consumption of 53 W max.

• The DFMS (Double Focusing
Magnetic mass Spectrometer) has a mass range from 1 amu to 150 amu and
a mass resolution of 3000 at 1 percent peak height. It is optimized for
very high mass resolution and a large dynamic range.

• The RTOF (Reflectron Time Of
Flight) mass spectrometer with a mass range from 1 amu to more than 300
amu and a high sensitivity.

COSIMA is a secondary ion mass
spectrometer equipped with a dust collector, a primary ion gun, and an
optical microscope for target characterization. Dust from the near
comet environment is collected on a target. The target is then moved
under a microscope where the positions of any dust particles are
determined. The cometary dust particles are then bombarded with pulses
of indium ions from the primary ion gun. The resulting secondary ions
are extracted into the time-of-flight mass spectrometer.

Science objectives: COSIMA will
perform in-situ measurements on individual dust particles emitted by
the target comet and collected by COSIMA dust collector subsystem. From
the resulting data it will be possible to determine:

• The elemental composition of
solid cometary particles to characterize comets in the framework of the
solar system chemistry

• The isotopic composition of
key elements in solid cometary particles such as H, C, Mg, Ca, Ti in
order to establish boundary conditions for models of the origin and
evolution of comets and thereby of the solar system

• The chemical states of the elements

• Variations of the chemical and isotopic composition between individual particulate components

• Changes in composition that occur as functions of time ("short-term variations") and orbital position

• The variability of the
composition of different comets by comparing the results to those
obtained previously from comet Halley

• The presence of an organic component that is not associated with a rocky phase

• The molecular composition of the organic phase of the solid cometary particles

• The molecular composition of the inorganic phase of the solid cometary particles

• The chemical state of the organic matter characterized by its saturation degree oxidation state and bond types.

Instrument: The core of the
COSIMA instrument is a TOF (Time of Flight) secondary SIMS (Ion Mass
Spectrometer) equipped with a dust collector, a primary ion gun, and an
optical microscope (COSISCOPE) for target characterization. Once one of
the targets on the target wheel has been exposed to cometary dust it is
moved in front of the microscope and imaged under shallow angle
illumination provided by light emitting diodes. On-board image
evaluation detects the presence and location of dust particles with
diameters exceeding a few µm and calculates their position
relative to the target reference point. Once the presence of features
of interest is established, the target is moved in front of the mass
spectrometer. Three nanosecond duration pulses of indium-115 with an
energy of 10 keV and about 10 µm in diameter from the primary ion
gun hit the selected feature. Secondary ions from the cometary matter
are extracted by the SIL (Secondary Ion extraction Lens) into the TOF
section. After passing deflection plates for beam steering the ions
travel through a field free section. Next they pass a two stage
reflector, return through the drift section to the ion detector. Its
main element is a single stage microsphere plate, where the ions are
detected. The arrival time of each ion is measured with an accuracy of
about 2 ns.

Precision in
the timing of the primary ion pulses, the correct selection of the
dimensions and the voltages of the mass spectrometer and the accurate
measurement of the secondary ion flight time are needed to obtain high
mass resolution in the COSIMA instrument. A mass resolution of 2000 is
achieved for ions having a flight time of 16 µs, which occurs for
ion masses of above 28 Daltons (atomic mass units).

Figure 21: Photo of the COSIMA flight model (image credit: MPS)

Operations: Since launch and
commissioning, the instrument health has been monitored in checkouts
every 6 months. The instrument proved to be in good health during the
checkouts performed so far. Operations have included mechanism tests,
instrument calibration, ion source maintenance, and interference check
between COSIMA and other instruments. The instrument will be
re-commissioned in April and July 2014, just before the science phase
of Rosetta at comet 67P/Churyumov-Gerasimenko.

Status:

• Sept. 8, 2014: The COSIMA
team presented an image of the first dust grains collected by the
COSIMA instrument when Rosetta was at a distance of less than 100 km
from the nucleus of comet 67P/Churyumov-Gerasimenko. On Aug. 11, the
first of COSIMA’s 24 target plates were exposed to space. On Aug.
24, when the COSIMA team took a look at the image of the plate, they
saw a number of large dust grains from the comet on a target that had
been pristine when examined one week before. A first examination of the
plate indicates that the largest two grains are about 50 µm and
70 µm in width, comparable to the width of a human hair. 38)39)

• August 8, 2014: Now that
comet 67P/Churyumov-Gerasimenko is within reach, Rosetta’s mass
spectrometer COSIMA, is beginning to reach for cometary dust. 40)

• In
early April 2014, the project uploaded new software, and after
switching the COSIMA instrument off and then back on, the previous
tests proved successful and COSIMA was up and running with a fresh
memory. On their way to or from the spacecraft, these data travel about
40 minutes through the inner Solar System. 41)

MIDAS (Micro-Imaging Dust Analysis System)

MIDAS is designed to collect and
image dust particles collected in the vicinity around the comet. The
measurement principle is based on atomic force microscopy. This
technique allows for true three-dimensional imaging at nanometer-scale
resolution. The instrument is built to investigate the smallest grain
size fraction released from the comet's surface; it will also
investigate the structural complexity of grain cluster up to a few
µm. More than 60 collector facets, which can be individually
exposed into the dust stream, and a total of 16 imaging sensors,
guarantee continuous observations throughout the mission lifetime. 42)

Scientific objectives: Dust
particles emitted from comet nuclei form a major source of information
for the understanding of primitive matter in our Solar System. It
represents remnant material from the early times of the formation
comets, asteroids and planets some 4.5 billion years ago. The prime
scientific objective of the MIDAS experiment is to image the
micro-topography and micro-textural units of cometary dust particles;
this provides important information about the characteristics and
nature of these particles, for example, about the composition of their
primary building blocks. In addition, sub-features on clean crystal
surfaces provide insight into either the growth conditions (twinning,
screw dislocations) and/or storage environment conditions (dissolution
marks).

Following the mapping of single
particles with a resolution in the few nanometer range, many
statistical parameters describe the cometary environment. This
comprises the statistical evaluation of the collected particles
according to size, volume and shape, but also temporal and spatial
variations of the particle flux can be deduced.

In summary, MIDAS will meet the
scientific objectives that have been established for this instrument
when the following information can be obtained during the rendezvous
with the comet:

• 3D images of single particles with a resolution better than 10 nm

• Search for "very small particles" (10 nm)

• Search for evidence of euhedral (well-formed, sharp-faced) crystals

• Possible detection of ferro-magnetic minerals

• Size distribution of particles

• Variation of particle fluxes on time scales between hours and days.

The instrument was developed by an
international collaborative team led by IWF (Institut für
Weltraumforschung - Space Research Institute), Graz, Austria;
ESA/ESTEC, Noordwijk, The Netherlands; Physics Department of the
University of Kassel, Germany; Institute of Applied Systems Technology
of Joanneum Research, Graz, Austria; Austrian Research Centers
Seibersdorf (now Ruag Space, Vienna); Vienna University of Technology,
Vienna, Austria. PI: Mark Bentley, IWF, Graz, Austria. 43)44)

Instrument: Dust grains in the size
range from 4 µm down to 4 nm will be imaged in three dimensions
by means of atomic force microscopy (AFM). AFM makes use of tiny
physical forces (van der Waals, interatomic, magnetic, etc.) that act
on a sensor in closest distance to a surface.

The sensor is a 600 µm long
cantilever arm with an extremely sharp 7 µm long tip mounted
underneath. The sensor is controlled by a piezoelectric mechanical
system that scans above the surface and senses its topography. The dust
particles enter the instrument via a funnel penetrating the
spacecraft's hull and hit the collector surface. Sixty-four of these
targets (coated silicon facets) are mounted on the perimeter of the
dust collector wheel. The facet exposed to the dust stream is rotated
and presented to the microscope, which approaches the surface
automatically and starts the scanning (imaging) process.

In the test sequence of the MIDAS
instrument, which ran over five contact passes from the ground station
to the spacecraft, the following results were obtained:

- Complete electronics checkout

- Cover opening by a pyrolytic device

- Unlocking of all clamp mechanisms

- Movement of linear stage and approach mechanism out of launch position

- Verification of all motors and mechanisms

- Verification of all 16 sensors by resonance search

- Verification of the scanner by imaging calibration surfaces

- Initial characterization of mechanical noise environment.

The overall result of the MIDAS
commissioning phase and subsequent instrument checkout demonstrated
full functionality and performance of the instrument up to the moment
the spacecraft (and payload) was put into hibernation in June 2011.

CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission)

CONSERT is a time domain
transponder that operates between one module that will land on the
comet surface and another that will orbit the comet. A radio signal is
transmitted from the orbiting component of the instrument and passes
through the comet nucleus to the component on the comet surface. The
signal is received on the lander, where some data is extracted, and
then immediately re-transmitted back to the orbiter, where the main
experiment data collection occurs. The variations in phase and
amplitude that occur as the radio waves pass through different parts of
the cometary nucleus will be used to perform tomography of the nucleus
and determine the dielectric properties of the nuclear material. 45)46)47)

The overall science objective of
the CONSERT investigation is to gather information about the
geometrical structure and electrical properties of the deep interior of
the comet nucleus. Inferences about the composition of the interior of
the comet will then be made from the measured electrical properties.
The main scientific objectives are:

• To measure the mean
dielectric properties and, through modelling, to set constraints on the
cometary composition (like material and porosity)

CONSERT probes the comet’s
interior by studying radio waves that are reflected and scattered by
the nucleus. The CONCERT instrument provides radio sounding and nucleus
tomography. The instrument's mass is limited to 3 kg. PI: Wlodek
Kofman, LPG (Laboratoire de Planetologie de Grenoble), CNRS/UJF,
Grenoble, France.

Instrument: CONSERT works as
a time domain transponder. The indirect and apparently complicated
transponding procedure reduces the required accuracy of the clocks on
the Orbiter and Lander, and makes it possible to stay within the
constraints on mass and power consumption imposed on the space
experiment. The CONSERT experiment on the orbiter and on the lander
both consist of a transmit/receive antenna and a transmitter and
receiver contained in a common box.

A 90 MHz radio signal, phase
modulated with pseudo-randomly encoded data is transmitted from the
orbiter towards the comet. The transmission lasts about 25 µs.
The signal propagates through the comet nucleus and is received on the
lander. The transmission cycle is repeated every 200 ms. The received
signal is digitised and accumulated in the lander in order to increase
the signal to noise ratio. Once the accumulation is finished, the
signal is compressed to obtain a time/space resolution corresponding to
100 ns which corresponds to about 20 m in the comet. After the signal
processing on the lander, which determines the position of the
strongest path, the lander transmits the same pseudo-random code with a
delay corresponding to that of the strongest path. The transmission
cycle again lasts about 25 microseconds. The signal propagates back to
the orbiter along virtually the same path, since the orbiter does not
travel far during the measurement cycle. The signal is received on the
orbiter, accumulated and stored in the memory in order to be sent to
Earth. A complete measurement cycle lasts about 1 s.

GIADA (Grain Impact Analyzer and Dust Accumulator)

GIADA will measure the number,
mass, momentum and velocity distribution of dust grains in the
near-comet environment. GIADA will analyze both grains that travel
directly from the nucleus to the spacecraft and those that arrive from
other directions having had their ejection momentum altered by solar
radiation pressure. 48)49)50)

• Dust flux measurement for
"direct" and "reflected" grains: Two populations of cometary grains
exist: "direct" (coming directly from the nucleus) and "reflected"
grains (coming from the Sun direction, under the action of the solar
radiation pressure). The two populations undergo very dissimilar
dynamic evolution in the coma and have different times of ejection from
the nucleus. In the case of Rosetta, "direct" and "reflected" grains
can be collected simultaneously. The relative amount will depend on the
probe position along its orbit. GIADA will be able to monitor grain
fluxes coming from different directions and will allow, for the first
time, discrimination between the two dust populations. This task is
fundamental to the determination of the original dust size
distribution. In turn, this information is required to define the dust
mass loss rate.

• Analysis of the dust
velocity distribution : The dust ejection velocity depends both on the
grain size and on time. Moreover, grains with a given size have a wide
dust velocity distribution. GIADA will allow the measurement of scalar
velocity and momentum for grains coming from the nucleus direction so
as to give mass and impact velocity of each analyzed "direct" grain.
From this information it will be possible to derive grain mass and
ejection velocity from the nucleus surface. For the first time we will
obtain:

- the size dependence of the dust ejection velocity

- the relation between most probable dust velocity and dust mass

- the velocity distribution for each dust mass

- the link between velocity dispersion and dust mass.

• Study of dust evolution in
the coma : Once ejected from the nucleus, grains may change their
physical properties due to several processes, including, for example,
fragmentation. These modifications may alter the grain size
distribution. The size distribution of grains collected by GIADA in the
nucleus direction should not be affected by the dust velocity
dispersion. Thus, changes in the dust distribution at different nucleus
distances can be linked directly to actual variations in the dust size
distribution and correlation can be found with dust fragmentation
and/or with emission from active areas on the nucleus.

• Correlation of dust changes
with nucleus evolution and emission anisotropy : The dust environment
characteristics depend on the comet-Sun distance and on the time
evolution of the nucleus. The continuous monitoring by GIADA of dust
flux and dynamic properties will offer the best opportunity to
characterize the time evolution of the dust environment as a function
of heliocentric distance. Nucleus imaging will allow us to link
observed changes to the nucleus evolution and to its spin state.

• Determination of dust to gas
ratio : One of the crucial parameters characterizing the comet nucleus
is the dust to gas ratio. Dust flux monitoring by GIADA is needed to
estimate the dust to gas ratio. This will be possible in combination
with results of other experiments.

• Other objectives : The data
provided by GIADA about dust fluxes and grain dynamic properties are
very important for the correct interpretation of images of the coma and
nucleus and mass spectrometer data. GIADA will help in the selection of
the surface science package landing site. The characterization of dust
emitting areas, and possibly of the dust population of different active
areas, will be necessary for the site selection process to achieve a
proper balance between safety and scientific interest.

GIADA will play an important role
for the health and the safety of various experiments and the spacecraft
itself, as it will be able to provide information about dust flux in
several directions. Optical surfaces of experiments and other devices
pointing to the nucleus will be polluted by the dust flux. GIADA data
will allow the prediction of deposition rates and informed decision
making for mission planning and operations. Data from GIADA will be the
only resource to predict and allow control of the performance
degradation of critical devices such as passive radiators and solar
panels.

Instrument:
The instrument comprises three modules: GIADA 1 measures momentum,
scalar velocity and mass of single grains entering the instrument by
the GDS (Grain Detection System) and the IS (Impact Sensor), placed in
cascade. The GIADA 2 module contains the MBS (Micro Balances System);
it controls the acquisition of data from the sensors and the operation
of the other subsystems. It also provides the power supply for the
whole experiment. The GIADA 3 module measures the cumulative dust flux
and fluence from different directions by means of five microbalances.
One microbalance points towards the nucleus, while the other four cover
the widest possible solid angle. 51)

In the GDS, four laser diodes with their foreoptics are used to form a thin (3 mm) light curtain (100 cm2).
For each grain passing through it, the scattered/reflected light is
detected by two series of four detectors (photodiodes) placed at
90º with respect to the sources. In front of each photodiode a
Winston cone is placed to achieve a uniform sensitivity in the
detection area.

The IS is a thin (0.5 mm) aluminum square diaphragm (sensitive area 100 cm2)
equipped with five piezoelectric sensors, placed below the corners and
its center. When a grain impacts the sensing plate, flexural waves are
generated on the plate, and are detected by the piezoelectric crystals.
The maximum displacement of these systems is directly proportional to
the impulse imparted, and the displacement of the crystal produces a
proportional potential. Through calibration, a known impulse may be
equated with a specific charge produced on the electrodes of the PZT
crystals. The detected signal is proportional to the momentum of the
incident grain through the factor (1+e), where e is the coefficient of
restitution.

When a grain enters GIADA 1, the
GDS gives a first estimate of the grain speed and starts a time counter
that is stopped when the IS detects the grain impact and the momentum
is measured. In this way, for each entering grain, speed, TOF (Time of
Flight), momentum and, therefore, mass are measured.

The microbalances in GIADA 3 each
consist of two quartz crystals oscillating at a frequency of about 15
MHz, one acting as a sensor, the other as a reference. The measured
physical quantity is the beat frequency between the two crystals. The
resonance frequency of the sensor quartz oscillator, exposed to the
dust environment, changes due to the variation of its mass as a result
of material accretion, while the reference crystal is not exposed to
the dust flux. Thus, the output signal is proportional to the mass
deposited on the sensor and dust flux and fluence are measured in time.
The use of a reference crystal ensures extremely small dependence on
temperature and power supply fluctuations and, thus, high sensitivity.

Figure 23: Two views of the GIADA instrument (image credit: INAF)

RPC (Rosetta Plasma Consortium)

Six sensors measure the physical
properties of the nucleus; examine the structure of the inner
coma;monitor cometary activity; and study the comet’s interaction
with the solar wind. The RPC assembly consists of:

The RPC is intended to investigate the following scientific areas of interest: 52)53)

• The physical properties of
the cometary nucleus and its surface. Emphasis will be given to
determination of the electrical properties of the crust, its remnant
magnetization, surface charging and surface modification due to solar
wind interaction, and early detection of cometary activity.

• The inner coma structure,
dynamics, and aeronomy. Charged particle observation will allow a
detailed examination of the aeronomic processes in the coupled
dust-neutral gas-plasma environment of the inner coma, its
thermodynamics, and structure such as the inner shocks.

• The development of cometary
activity, and the micro- and macroscopic structure of the solar-wind
interaction region as well as the formation and development of the
cometary tail.

In order to realize these
investigations extensive in-situ monitoring of the plasma electrons and
ions, their composition, distribution, temperature, density, flow
velocity, and the magnetic field will be necessary. These measurements
will improve the understanding of the coupling processes of cometary
dust, gas, and plasma as well as its interaction with the solar wind.
The plasma and fields measurements thus provide complementary
information to that of other Rosetta instruments for a deeper
understanding of the overall physics and chemistry of an active comet.

The flybys of asteroid Steins and
asteroid Lutetia have provided an opportunity to study in detail the
physics of the solar wind - asteroid interaction. RPC has excellent
capabilities for the investigation of this interaction. It has also
been possible to study the magnetic and electric conductivity
properties of the asteroids.

ICA (Ion Composition
Analyzer): ICA measures the three-dimensional velocity distribution and
mass distribution of positive ions. The mass resolution is sufficient
to differentiate between the major particle species such as protons,
helium, oxygen, molecular ions, and heavy ion clusters (dusty plasma).
The ICA comprises an electrostatic arrival angle filter, a
hemispherical electrostatic analyzer employed as an energy filter, and
a magnetic deflection momentum filter. Particles are detected using a
large micro channel plate and a two-dimensional anode array.

IES (Ion and Electron
Sensor): The IES will simultaneously measure the flux of electrons and
ions in the plasma surrounding the comet over an energy range from
around one electron volt, which approaches the limits of detectability,
up to 22 keV. IES consists of two electrostatic analyzers, one for
electrons and one for ions, which share a common entrance aperture. The
charged particle optics for IES employs a toroidal top-hat geometry
along with electrostatic angle deflectors to achieve an
electrostatically scanned field of view of 90º x 360º. 54)

LAP (Langmuir Probe): The
LAP instrument will measure the density, temperature and flow velocity
of the cometary plasma. It comprises two spherical sensors mounted at
the tip of deployable booms, with the sensors capable of being swept in
potential to measure the current-voltage characteristic of the
intervening plasma, which provides information on the electron number
density and temperature. The probes can be held at a fixed bias
potential to measure plasma density fluctuations and by a
time-of-flight analysis of the signals from the two probes the plasma
flow velocity can be determined. 55)

MAG (Flux Gate
Magnetometer): The MAG will measure the magnetic field in the region
where the solar wind plasma interacts with the comet. It consists of
two triaxial fluxgate magnetometer sensors mounted on a 1.5 m
deployable boom that points away from the comet nucleus. One sensor is
mounted near the outboard tip of the boom and one is mounted part way
along the boom. The use of two sensors allows the effects of the
spacecraft's own magnetic field to be minimized. MAG will also study
any magnetic field possessed by the comet nucleus, in cooperation with
the ROMAP magnetometer experiment on the Rosetta lander.

MIP
(Mutual Impedance Probe): MIP will derive the electron gas density,
temperature, and drift velocity in the inner coma of the comet by
measuring the frequency response of the coupling impedance between two
dipoles. MIP will also investigate the spectral distribution of natural
waves in the 7 kHz to 3.5 MHz frequency range and monitor the dust and
gas activity of the nucleus.

PIU (Plasma Interface Unit):
PIU acts as an interface between the five instruments that make up RPC
and the Rosetta spacecraft by providing a single path for the
transmission of scientific and housekeeping data to the ground and for
the receipt and processing of commands sent from the ground. The PIU
also takes power from the spacecraft and converts, conditions and
manages it for the RPC instruments. PIU also performs on-board data
processing for the MAG sensor unit, which has no data processing
capability of its own. 56)

RSI (Radio Science Investigation)

RSI makes use of the communication
system that the Rosetta spacecraft uses to communicate with the ground
stations on Earth. Either one-way or two-way radio links can be used
for the investigations. In the one-way case, a signal generated by an
ultra-stable oscillator on the spacecraft is received on Earth for
analysis. In the two way case, a signal transmitted from the ground
station is transmitted back to Earth by the spacecraft. In either case,
the downlink may be performed in either X-band or both X-band and
S-band. — PI: Martin Pätzold, University of Cologne,
Germany. 57)

The goal of RSI is to investigate
the nondispersive frequency shifts (classical Doppler) and dispersive
frequency shifts (due to the ionized propagation medium), the signal
power and the polarization of the radio carrier waves. Variations in
these parameters will yield information on the motion of the
spacecraft, the perturbing forces acting on the spacecraft and the
propagation medium.

Science objectives: Doppler data
provide time-resolved measurements of the spacecraft motion and the
plasma state and thus may be used for physical investigation of the
nucleus and the inner coma of the comet. In particular, the following
scientific objectives may be addressed by an analysis of dual-frequency
one-way or two-way radiometric tracking data, together with information
provided by other Rosetta experiments, for example the remote imaging
system (OSIRIS):

Instrument: The two-way
radio link is established by transmitting an uplink radio signal either
at S-band or X-band to the spacecraft. The received uplink carrier
frequency is transponded to downlinks at X-band and S-band upon
multiplying by the constant transponder ratios 240/221 and 880/221,
respectively, in order that the ratio of the two downlinks is 880/240 =
11/3. This radio mode takes advantage of the superior frequency
stability inherent to the hydrogen maser in the ground station on
Earth. This mode is used for all RSI gravity science applications,
routine tracking observations when in orbit during the escort phase,
and for the sounding of the solar corona.

The one-way radio link is used only
during an occultation of the spacecraft by the nucleus as seen from
Earth. This enables radio sounding of the immediate vicinity of the
nucleus and perhaps even the nucleus itself, should the solid cometary
body prove to be penetrable by microwaves. These one-way occultation
experiments require an USO (Ultra-Stable Oscillator) added to the radio
subsystem. The prime purpose of the USO is to serve as a phase-coherent
frequency reference for the simultaneous one-way downlink transmissions
at S-band and X-band. The required stability (Allan variance) of the
USO is about Δf/f ~10-13 at 10-1000 seconds
integration time. The one-way radio link can be transmitted either
while receiving a non-coherent uplink or without any uplink contact at
all.

Ground segment: Ground
stations include antennas, associated equipment and operating systems
in the tracking complexes of Perth (ESA, 35 m), Australia, and the DSN
(Deep Space Network) of NASA, (34 m) in California, Spain and
Australia. A tracking pass consists of typically eight to ten hours of
visibility. Measurements of the spacecraft range and carrier Doppler
shift can be obtained whenever the spacecraft is visible. In the
two-way mode the ground station transmits an uplink radio signal at
S-band (if the spacecraft receiver operates at S-band) or at X-band and
receives the dual-frequency simultaneous downlink at X-band and S-band.
The information about signal amplitude, received frequency and
polarization is extracted and stored as a function of ground receive
time.

SREM (Standard Radiation Environment Monitor)

In addition to these scientific
experiments the orbiter is also equipped with a SREM device to monitor
the high energetic, ionizing particle environment aboard the
spacecraft. The objective of SREM is to provide a continuous, almost
uninterrupted measurement of the high energetic particles encountered
by Rosetta and provide this information for mission analysis purposes.

Philae (Rosetta Lander)

The Rosetta lander Philae can be
considered a scientific spacecraft of its own that is carried and
delivered by the Rosetta orbiter to the comet. Upon proposal by various
scientists, lead by Helmut Rosenbauer from the MPS (Max Planck
Institute for Solar System Research) in Katlenburg-Lindau, the 10
scientific instruments and the various spacecraft subsystems are
provided by a consortium of spaceflight agencies and research
institutes from 6 European countries and by ESA. The Philae lander is
provided by a consortium led by DLR, MPS, CNES and ASI. DLR played a
major role in building the lander and runs the LCC (Lander Control
Center) at DLR Cologne, which is preparing for and overseeing the
difficult task of landing on the comet, a feat never before
accomplished. 58)59)60)61)62)63)

The goal of Philae's mission is to
land successfully on the surface of a comet, and transmit data from the
surface about the comet's composition. The scientific goals of the
mission focus on "elemental, isotopic, molecular and mineralogical
composition of the cometary material, the characterization of physical
properties of the surface and subsurface material, the large-scale
structure and the magnetic and plasma environment of the
nucleus.”

The SONC
(Science Operations and Navigation Center) is located at CNES in
Toulouse, France. Both centers are directly connected to the RMOC
(Rosetta Mission Operations Center) at ESOC, Darmstadt. The Rosetta
science operations planning is performed at the RSGS (Rosetta Science
Ground Segment) at ESAC, near Madrid. - The responsibility for the
Lander delivery lies at ESA. However, close cooperation between the
partners is envisaged, to reach the challenging task of the first
successful landing on a comet. 64)

The Philae lander is designed to
touch down on the comet's surface after being deployed from the main
spacecraft body and "falling" from a height of 25 km at about 1 m/s
towards the comet along a ballistic trajectory. Upon contact, it will
deploy two harpoons to anchor itself to the surface, and the legs are
designed to dampen the initial impact to avoid bouncing, because the
comet's escape velocity is only around 0.5 m/s.

Communications
with Earth will use the orbiter spacecraft as a relay station to reduce
the electrical power needed. The mission duration on the surface is
planned to be at least one week, but an extended mission lasting months
is possible.

Rosetta-Philae RF link: The
transceiver is a full duplex S-band transmission set for digital data
developed specifically for space applications. The conception made by
Syrlinks was done with drastic objectives for mass and power
consumption. For this, the use of commercial parts was decided leading
to a low cost product widely used afterwards on the Myriade platform
family. The transceiver is composed of a transmitter, a receiver and a
reception filter for dual antenna use (Figure 25).
The filter protects the receiver from out-of-band signals, particularly
from the transmitter. The two functions (receiver and transmitter) are
fully independent and can be activated separately. Technical details
are given in the Table 14 and an illustration in Figure 26. 65)66)

There are two
transceivers on both sides of the RF link. The redundancy is activated
with RF switches on orbiter side (1 Tx/1 Rx active) and with diplexer
on lander side (1 Tx/2 Rx active). The choice of implementing identical
RF chains for transmission and reception on the orbiter and the lander
has given great advantages, such as cutting procurement costs and
simplifying qualification, integration and testing.

With 1W RF output power and 1 dBi gain (@ 60º) patch antennas, link establishment is possible for distances up to 150 km.

The lander telecommunication system
answers to a request-to-send protocol from the orbiter at any time.
This handshake protocol, which implies full duplex equipment and which
was specifically designed for Rosetta mission ensures a desired quality
of transmission even when the relative geometry and visibility between
the orbiter and the lander is not favorable.

In the housekeeping telemetry
available at orbiter side, one parameter is particularly interesting to
get information beyond its intrinsic value: the RSSI (Received Signal
Strength Indicator). From this raw telemetry value, it is possible to
extract the received power level on orbiter side, which can be then
processed.

Lander bus: The main
structure of the lander is made from carbon fiber, shaped into a plate
maintaining mechanical stability, a platform for the science
instruments, and a hexagonal "sandwich" to connect all the parts. The
main body rests on a tripod landing gear, with ice screws and sensors
integrated in the feet. All instruments and the drill are depicted in
their deployed configuration. The open face of Philae with instruments
exposed to the cometary environment is colloquially termed
“balcony”.

The total mass is about 100 kg. Its "hood" is covered with solar cells for power generation.

Battery assembly: The lander energy
storage is based on two types of sources: primary batteries for short
term activities (1000 Wh), secondary batteries for long term
activities(140 Wh). The assembly includes the associated electronics.

Figure 27: A schematic view of the Philae spacecraft with the extended lander system (image credit: Philae Team)

Legend to Figure 27:
The figure shows the overall structure, the subsystems and the
experiment compartment of the Lander. Some instruments are not visible
in the drawing: specifically, the instruments in charge of analyzing
the samples distributed by the SD2 (CIVA, COSAC, PTOLEMY), and the
down-looking camera (ROLIS).

Figure 28: Side view schematics
of the inner structure of the lander compartment showing the location
of COSAC and PTOLEMY systems, the CONSERT antennas, the SESAME dust
sensor and various CIVA cameras (image credit: Philae Team)

Science objectives:

The general tasks of Philae are to
get a first in situ analysis of primitive material from the early solar
system and to study the structure of a cometary nucleus which reflects
growth processes in the early solar system and to provide ground truth
for Rosetta Orbiter instruments. The scientific objectives of the
Lander are:

Legend of Figure 29:
The ejection maneuver takes place at an altitude on the order of 1 km
only; the Lander eject velocity partly cancels Rosetta’s orbital
velocity, such that Philae moves on an comet-surface crossing ellipse,
stabilized by a flywheel and the optional use of a cold-gas thrusters
(in z direction). After touchdown on the moving comet surface, the
cold-gas system is activated to provide a hold-down thrust until the
harpoons have safely anchored the Lander.

Philae sensor complement:

The Rosetta Lander carries a
further nine experiments, as well as a drilling system to take samples
of subsurface material. The Lander instruments are designed to study in
situ for the first time the composition and structure of the surface
and subsurface material on the nucleus. The science payload of the
lander consists of ten instruments with a mass of ~27 kg, making up
nearly one-third of the mass of the lander. 67)

Comet Nucleus Infrared and
Visible Analyzer: A group of six identical micro-cameras that take
panoramic images of the surface. A spectrometer studies the
composition, texture and albedo (reflectivity) of samples collected
from the surface. CIVA mass = 3.4 kg (sharing parts with ROLIS).

COmet Nucleus Sounding Experiment
by Radiowave Transmission: Perform tomography of the nucleus by
measuring electromagnetic wave propagation from the Rosetta orbiter
through the nucleus that are returned by a transponder on the Philae
lander in order to determine the comet's internal structure. CONCERT
mass = 1.8 kg.

Wlodek Kofman, LPG, CNRS/UJF, Grenoble, France

SD2

Sample Drill and Distribution
System: Obtains soil samples from the comet at depths of 0 to 230 mm
and distributes them to the Ptolemy, COSAC, and CIVA subsystems for
analyses. The SD-2 system contains four types of subsystems: drill,
carousel, ovens, and volume checker. SD2 mass of 4.8 kg.

Amalia Ercoli-Finzi, Politechnico di Milano, Milano, Italy

Table 15: The payload of the Rosetta lander Philae lead investigators: Jean-Pierre Bibring and Hermann Boehnhard

For the collection of samples and
the deployment of instruments it is important to note that the Lander
can be rotated around its z (vertical) axis by 360º defining a
“working circle” around the Lander body axis. Thus,
arbitrary locations can be accessed by the sampling drill (SD2), the
down-looking camera (ROLIS), and the APXS sensor; the MUPUS-PEN and the
SESAME-PP electrodes are attached to the latter two instruments. Also
the stereo camera pair of CIVA will be able to image a full panoramic
of 360º using the Lander rotation capability.

Note that the landing gear also
provides a tilting capability. This capability had to be drastically
reduced in range (to ±5º) after the change of target comet
in 2003 to ensure a safe landing.

Philae operations:

The science operations of Philae
are divided into various phases. During the 10 year cruise, check-ups,
calibrations, software and command up-loads are scheduled as well as
occasional observation campaigns for CIVA-P (flybys and ROMAP (flybys,
solar wind, comet tail crossings).

After arrival at the comet, global
mapping by the Orbiter instruments and the selection of a landing site,
the Separation–Descent–Landing phase begins. Immediately
before release from the Orbiter, thermal preparation and battery
charging are foreseen. Immediately after the eject, the Landing Gear is
unfolded, thereby releasing the CONSERT antennas. Then, the ROMAP boom
is deployed. A telemetry contact to the Orbiter will be established a
few minutes after release until well after landing. During descent
(30–60 min) to the comet’s surface, scientific measurements
(images by ROLIS, magnetic field measurements by ROMAP-MAG, dust impact
by SESAME-DIM and -CASSE, calibrations of SESAME-CASSE and MUPUS-TM)
will be performed to monitor the cometary environment between the
Orbiter and the surface of the nucleus, to observe the nucleus while
approaching, to characterize remotely the landing site and to document
the touchdown event of the Lander at the surface. ROLIS descent images
will be taken until touchdown and MUPUS-ANC measurements during the
actual anchoring.

During the “first science
sequence” of approximately five days, Philae will be operated
mainly on primary batteries, thus minimizing sensitivity to landing
geometry (solar irradiance of the cells). In the first 60 hours
following the touch-down, all instruments will work in their baseline
mode at least once at full completion of their relevant science goals.
In particular a full panorama of the landing site will be taken by
CIVA-P immediately after landing and cometary samples will be acquired
by SD2, both from the surface and from the maximum depth reachable with
drill (i.e., about 0.2 m); these samples will then be processed by the
relevant instruments (COSAC, Ptolemy, CIVA-M). MUPUS-PEN and APXS will
be deployed and thermal conductivity, thermal diffusivity, strength
measurements be made by MUPUS and the first X-ray and alpha spectra
will be recorded by APXS. CONSERT will sound the nucleus over at least
one full Orbiter orbit relative to the Lander. ROMAP will observe the
daily variation of magnetic field and the plasma properties. All three
parts of SESAME will perform measurements (PP only after MUPUS-PEN
and/or APXS have been deployed). The Lander resources should enable at
least a partial redo of this sequence over the following 60 hours, if
partial failure (e.g., in data transmission) had happened. If performed
successfully, the first sequence will secure a “minimum science
success” of the Lander mission. 68)

During the “long-term science
mission” (up to three months until r = 2 AU is reached) all
instruments will be operated mostly sequentially, powered by the solar
cells and buffered by the secondary (rechargeable) batteries. The
Lander has enough flexibility to allow—by rotation around its
body axis—the optimized orientation of the solar cells with
respect to the local time, to drill several boreholes, and to measure
physical properties all around the landing site.

The data volume to be uplinked to
Earth is 235 Mbit during descent and the first five days, and 65 Mbit
during each subsequent 60 hour period. However, depending on actual
telemetry coverage and Orbiter requirements, a significantly larger
data volume is expected.

With the current best estimate of
the comet environment, about 52–65 hours of primary mission
operations are feasible (incl. a 30% system margin). Primary power
during the first science sequence is 15–20 W; the solar cells
generate 10 W during the day at 3 AU.

The long-term operations then rely
entirely on the solar generator; the end of life will be determined
either by overheating (the thermal system is designed for a range of
2–3 AU) or by insufficient power if the solar cell degradation
(mainly by dust deposition) becomes too severe.

• August 12, 2019: Last week
marked five years since ESA’s Rosetta probe arrived at its
target, a comet named 67P/Churyumov-Gerasimenko (or 67P/C-G). Tomorrow,
13 August, it will be four years since the comet, escorted by Rosetta,
reached its perihelion – the closest point to the Sun along its
orbit. This image, gathered by Rosetta a couple of months after
perihelion, when the comet activity was still very intense, depicts the
nucleus of the comet with an unusual companion: a chunk of orbiting
debris (circled). At that time, the spacecraft was at over 400 km away
from the comet's center. 70)

Figure 31: Animated sequence of
images obtained by ESA's Rosetta probe at Comet
67P/Churyumov-Gerasimenko on 21 October 2015. The sizeable chunk in
this view was spotted by astrophotographer Jacint Roger from Spain, who
mined the Rosetta archive, processed some of the data, and posted the
finished images on Twitter as an animated GIF. Scientists at ESA and in
the OSIRIS instrument team are now looking into this large piece of
cometary debris in greater detail. Dubbed a ‘Churyumoon’ by
researcher Julia Marín-Yaseli de la Parra, the chunk appears to
span just under 4 m in diameter. - Modelling of the Rosetta images
indicates that this object spent the first 12 hours after its ejection
in an orbital path around 67P/C-G at a distance of between 2.4 and 3.9
km from the comet’s center. Afterwards, the chunk crossed a
portion of the coma, which appears very bright in the images, making it
difficult to follow its path precisely; however, later observations on
the opposite side of the coma confirm a detection consistent with the
orbit of the chunk, providing an indication of its motion around the
comet until 23 October 2015. Scientists have been studying and tracking
debris around 67P/C-G since Rosetta’s arrival in 2014. The object
pictured in this sequence is likely the largest chunk detected around
the comet, and will be subject to further investigations [image credit:
ESA/Rosetta/MPS/OSIRIS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/J. Roger (CC
BY-SA 4.0)]

- Comet 67P/C-G is a dusty object. As it neared its closest approach to the Sun
in late July and August 2015, instruments on Rosetta recorded a huge
amount of dust enshrouding the comet. This is tied to the comet’s
proximity to our parent star, its heat causing the comet’s
nucleus to release gases into space, lifting the dust along. Spectacular jets were also observed, blasting more dust away from the comet. This disturbed, ejected material forms the ‘coma’, the gaseous envelope encasing the comet’s nucleus, and can create a beautiful and distinctive tail.

- A single image from Rosetta’s OSIRIS instrument can contain hundreds of dust particles and grains
surrounding the 4 km-wide comet nucleus. Sometimes, even larger chunks
of material left the surface of 67P/C-G – as shown here.

- The sizeable chunk in this view
was spotted a few months ago by astrophotographer Jacint Roger from
Spain, who mined the Rosetta archive, processed some of the data, and
posted the finished images on Twitter as an animated GIF. He spotted
the orbiting object in a sequence of images taken by Rosetta’s
OSIRIS narrow-angle camera on 21 October 2015. At that time, the
spacecraft was at over 400 km away from 67P/C-G’s center. The
animated sequence is available for download here.

• April 26, 2019:
Two-and-a-half years have passed since the operational phase of the
Rosetta mission came to an end in September 2016. However, scientific
evaluation of the enormous amounts of data from the instruments on the
spacecraft and the Philae lander is still ongoing. The team of
scientists working on the VIRTIS instrument have now published new
findings relating to the surface temperature and thermal effects on the
'duck-shaped' Comet 67P / Churyumov-Gerasimenko, Germany's scientific
contributions to VIRTIS are led by the German Aerospace Center (DLR). 71)72)

- The Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS)
acquired infrared images of the comet from on board the Rosetta orbiter
during August and September 2014, approximately one year before the
comet reached perihelion – the point in its orbit closest to the
Sun. During the period under consideration, the comet was still distant
from the Sun, and its level of activity was low. The researchers
converted the images into thermal maps.

- Temperature is the most important
parameter for deriving the gas and dust activity typical of comets.
First, the VIRTIS team measured the average temperature of the comet's
nucleus on its daytime side. While the average surface temperature over
the two months was approximately minus 60ºC, the scientists also
identified places that were significantly warmer, at around minus
43ºC. These included a cavity in the surface, where the inner
walls reflected the thermal radiation and thus led to stronger warming,
referred to as self-heating.

- Self-heating also occurs at the
'duck's neck' connecting the two lobes of the comet. Temperatures were
higher here than the laws of black-body radiation would imply. Assuming
a dust-dominated surface a few millimeters thick and minimal
sublimation of volatile substances, self-heating is attributable to
surface roughness. The self-heating effect is enhanced by the striking
concave shape of the 'neck'.

- Another
significant finding concerns the thermal gradients caused by sudden
shadows cast alternately onto the 'neck' by the two lobes of the comet
during solar illumination. These localized shadows on the 'neck'
created extreme temperature differences within the space of just a few
minutes, which might be 10 times greater than normal daily variations
in temperature on other areas of the surface. "To better investigate
seasonal temperature effects on the nucleus, we concentrated on a
region named Imhotep, which is relatively flat and far from the 'neck',
and where the self-heating effect is significantly lower," says
Gabriele Arnold of the DLR Institute of Planetary Research. "For this
area, we compared the observations performed by VIRTIS with those of
the Microwave Instrument for the Rosetta Orbiter (MIRO), another
instrument on board Rosetta. MIRO made it possible to measure the
temperature in larger depressions on the comet. The findings of the two
instruments can be explained by the theory that the Imhotep region has
a thin surface layer consisting mainly of loose dust."

- Imhotep was also observed a few
months later, when the comet was much closer to the Sun. The
temperature values obtained by VIRTIS were much higher than before, but
lower than expected, given that the scientists were working on the
assumption that the surface layer consisted only of loose dust. This
led the researchers to conclude that the composition of the uppermost
layers must have changed over time. The quantity of volatiles within it
must have increased, resulting in a higher degree of sublimation and
more intense comet activity. This in turn can cause surface
temperatures to be lower than would be reached by a layer consisting
solely of dust.

- All the observational evidence
suggests a comet nucleus with thermal behavior that is dominated by
phenomena associated with the morphology and chemical and physical
state of the thin uppermost surface layer, which is only a few
centimeters thick. In the subsurface, the nucleus is thought to remain
essentially unchanged and has only been weakly influenced by previous
approaches to the Sun.

- Gabriele Arnold sums up: "The work
that has just been published shows that the ongoing evaluation of the
large quantity of data acquired will continue to provide unique
findings for comet research and the study of the early Solar System,
even years after the end of the Rosetta mission."

• April 23, 2019: From a
distance of five million kilometers to within 20 meters, ESA’s
Rosetta spacecraft captured images of Comet 67P/Churyumov-Gerasimenko
from all angles. 73)

- Between the first and the last
images lies one of humankind’s greatest space adventures to
rendezvous with and follow a comet as it orbited the Sun, and deploy a
lander to its surface.

- Figure 33
is just one of almost 70,000 images taken with Rosetta’s
high-resolution imaging system OSIRIS that are now available via a new
online and mobile-friendly ‘comet viewer’
created in a joint project with the Department of Information and
Communication at Flensburg University of Applied Sciences, and the Max
Planck Institute for Solar System Research, who lead the OSIRIS team.

- The image viewer hosts the full
archive, but also has subsections organizing image sets into themes:
for example, images showing towering cliffs and bizarre cracks on the
comet surface, or those focusing on spectacular dust fountains as the
comet launched gas and dust jets into space as its surface ices were
warmed as it came closer to the Sun on its orbit.

- The
collection of OSIRIS images captured the farewell of lander Philae as
it dropped towards the surface of the comet, and later, towards the end
of the mission, the feverish search for the hidden robot.

- Within the new comet viewer, each
of the nearly 70,000 images is supplemented with the date on which it
was taken, the distance to the comet, and a short accompanying text
briefly describing what is seen in the image. The images can be
downloaded in full resolution and can also be directly shared to
Twitter and Facebook.

- For users who wish to delve deeper
or use the archive for research purposes, the images are also available
in scientific data format; in addition, there is information available
on the filters used, focal lengths, and exposure times as well as
references to the scientific documentation and evaluation software.

Figure 33:
Seen from afar, the comet is usually likened to a duck in shape, but in
this enchanting close-up view its profile resembles that of a
cat’s face seen side-on. The two ‘ears’ of the cat
make up the twin peaks either side of the ‘C. Alexander
Gate’ – named for US Rosetta Project Scientist Claudia
Alexander who passed away in July 2015. These impressive cliffs lie at
the border between the Serqet and Anuket regions on the comet’s
head. The image was taken on 6 October 2014 from a distance of 18.6 km
to the comet [image credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (CC BY-SA 4.0)]

• April 12, 2019: ESA’s Rosetta
mission was an incredible feat of science and engineering. Studying a
comet up close, and even landing a probe on its surface, was an apt
demonstration of ESA’s abilities. Even though the mission ended
in 2016, its benefits live on in the ESA Academy’s Rosetta
Science Operations Scheduling Legacy Workshops. The 2019 workshop has
just been completed by 30 university students from 12 different ESA Member States and Canada. 74)

- "The friendly and international
environment, together with the knowledge acquired during these four
days were the best parts of the workshop,” reviewed a Spanish
student from the Polytechnic University of Catalonia. “This
workshop was one of the few opportunities that I have had, as a
student, to meet international science operation experts with the same
interests and motivations and a common goal, while learning from their
shared experiences in a mission that every space technology student is
very fond of. I also found important the fact that this expert-student
flow of information did not happen only during the lectures, but also
in less formal scenarios, such as over coffee breaks or dinner. The
workshop exceeded all my expectations and I had no idea how much I
could learn in just four days. What an amazing week!"

- The students were excited to learn
about science operations scheduling, and how it was planned for the
Rosetta mission. A combination of lectures and exercises using the
ESA’s Mission Analysis and Payload Planning Software (MAPPS) kept each day varied.

- The students were divided into
groups of three, each with a different background. They were supported
by the experts that had scheduled the Rosetta mission. Various
scenarios were proposed, such as Rosetta’s arrival at the comet,
and the search for the Philae lander after it bounced on the
comet’s surface. These situations provided context, and gave
ample opportunity to both utilize the planning software, and draw upon
the experience of the experts. In one scenario, students had to
schedule the pictures that needed to be taken for Philae’s
descent, and then scour the Rosetta archive to find and analyze them.

- "During this week I embarked on a
journey with Rosetta, alongside the people that dedicated their careers
to see this mission succeed,” said a Portuguese student from the
Delft University of Technology. “From arriving at the comet to
its final descent, Rosetta unveiled the secrets of 67P, and produced
invaluable science during its lifetime. Hopefully the students involved
in the workshop will be the future scientists and engineers exploring
our Solar System and beyond, following the footsteps of Rosetta!"

- The accompanying lectures were
designed to support and complement the exercises. They also allowed the
students to gain insight into the many and varied aspects of the
Rosetta mission. Some were broad, such as an overview of Rosetta; while
others were more specific, such as planning the end of mission
operations.

- Throughout the week, each student
group had the opportunity to present the results of an exercise to the
rest of the participants. This allowed them in-depth discussions, which
were evaluated so that the students could claim ECTS credit(s) on their return to their respective universities. In addition, the students enjoyed a guided tour of ESEC-Redu and the PROBA control and operations rooms, which proved to be particularly illuminating.

Figure 34: This second edition of the workshop was held from 2 to 5 April 2019 at ESA Academy’s Training and Learning Facility
in ESEC-Galaxia, Belgium.. Present to share expertise with the students
were seven experts, scientists and engineers, from ESA and the Max Planck Institute for Solar System Research.
Among them were the Rosetta Project Scientist and Spacecraft Operations
Manager. Personally involved with the Rosetta mission, the trainers had
unrivalled knowledge and invaluable viewpoints, and they peppered the
week with stories and anecdotes (image credit: ESA)

- A Spanish student from the
University Carlos III of Madrid found this to be a formative
experience: “There was T. S. Eliot quote I read a long time ago,
which goes ‘Only those who risk going too far can possibly find
out how far one can go’. I have always tried to follow that
ideal. The second edition of the Rosetta Science Operations Scheduling
Legacy Workshop proved me that ESA not only follow that ideal, but also
achieve it. I truly wish one day I will work at ESA.”

Figure 35: Photo of the students and trainers at the ESA Academy's Training Center (image credit: ESA)

• February 18, 2019: Feeling
stressed? You’re not alone. ESA’s Rosetta mission has
revealed that geological stress arising from the shape of Comet
67P/Churyumov–Gerasimenko has been a key process in sculpting the
comet's surface and interior following its formation. 75)

Figure 36:
Single frame enhanced NavCam image taken on 27 March 2016, when Rosetta
was 329 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The
scale is 28 m/pixel and the image measures 28.7 km across (image
credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0)

- Small, icy comets with two distinct lobes seem to be commonplace in the Solar System, with one possible mode of formation a slow collision of two primordial objects
in the early stages of formation some 4.5 billion years ago. A new
study using data collected by Rosetta during its two years at Comet
67P/C-G has illuminated the mechanisms that contributed to shaping the
comet over the following billions of years.

- The researchers used stress modelling and three-dimensional analyses of images taken by Rosetta’s high resolution OSIRIS camera to probe the comet’s surface and interior.

- “We found networks of faults
and fractures penetrating 500 meters underground, and stretching out
for hundreds of meters,” says lead author Christophe Matonti of
Aix-Marseille University, France.

- “These geological features
were created by shear stress, a mechanical force often seen at play in
earthquakes or glaciers on Earth and other terrestrial planets, when
two bodies or blocks push and move along one another in different
directions. This is hugely exciting: it reveals much about the
comet’s shape, internal structure, and how it has changed and
evolved over time.”

- The model developed by the
researchers found shear stress to peak at the center of the
comet’s ‘neck’, the thinnest part of the comet
connecting the two lobes.

- “It’s as if the
material in each hemisphere is pulling and moving apart, contorting the
middle part – the neck – and thinning it via the resulting
mechanical erosion,” explains co-author Olivier Groussin, also of
Aix-Marseille University, France.

- “We think this effect
originally came about because of the comet’s rotation combined
with its initial asymmetric shape. A torque formed where the neck and
‘head’ meet as these protruding elements twist around the
comet’s center of gravity.”

- The observations suggest that the
shear stress acted globally over the comet and, crucially, around its
neck. The fact that fractures could propagate so deeply into 67P/C-G
also confirms that the material making up the interior of the comet is
brittle, something that was previously unclear.

- "None of our observations can be
explained by thermal processes,” adds co-author Nick Attree of
the University of Stirling, UK. “They only make sense when we
consider a shear stress acting over the entire comet and especially
around its neck, deforming and damaging and fracturing it over billions
of years.”

- Sublimation, the process of ices
turning to vapor and resulting in comet dust being dragged out into
space, is another well-known process that can influence a comet’s
appearance over time. In particular, when a comet passes closer to the
Sun, it warms up and loses its ices more rapidly – perhaps best
visualized in some of the dramatic outbursts captured by Rosetta during its time at Comet 67P/C–G.

- The new results shed light on how dual-lobe comets have evolved over time.

Figure 37: These images show how
Rosetta’s dual-lobed comet, 67P/Churyumov-Gerasimenko, has been
affected by a geological process known as mechanical shear stress. The
comet’s shape is shown in the left two diagrams from top and side
perspectives, while the four frames on the right zoom in on the part
marked by the overlaid black box (the comet’s
‘neck’). The red arrow points to the same spot in both
images, seen from a different perspective. - The two central frames
show this part of the neck as imaged by Rosetta’s OSIRIS camera,
and used in a new study exploring how the comet’s shape has
evolved over time. The two frames on the right highlight different
features on the comet using these images as a background canvas. Red
lines trace fracture and fault patterns formed by shear stress, a
mechanical force often seen at play in earthquakes or glaciers on Earth
and other terrestrial planets. This occurs when two bodies or blocks
push and move along one another in different directions, and is thought
to have been induced here by the comet’s rotation and irregular
shape. Green marks indicate terraced layers [image credit:
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA;
C. Matonti et al. (2019)] 76)77)

Legend to Figure 38:
The comet is thought to have formed this long ago in the primordial
disc of the Solar System, perhaps as two small objects slowly collided
and stuck together. Comets form in the icy outer Solar System and are
stored there in vast clouds before beginning their journey inwards;
comet 67P/C-G is thought to have entered the giant planet region
hundreds of thousands to millions of years of ago. By this point a form
of geological erosion named mechanical shear stress had taken hold, and
was the dominant process sculpting and shaping the comet’s
surface and interior. A new study using data from Rosetta found this
stress to peak in the region connecting the two lobes of the comet: the
‘neck’. This neck bore the brunt of mechanical erosion,
fracturing and thinning over time – as shown in the diagram by
the cross-hatched lines.

The final steps cover the time
period from tens of thousands of years ago to present day, a period
during which sublimation erosion was dominant in shaping the
comet’s surface and interior. This kind of erosion takes place as
the Sun warms ices within the comet, causing the ice to turn to gas and
escape to space, carrying cometary material along with it. This
weakened the comet’s neck further, and the force grew stronger as
it travelled inwards from Jupiter’s orbit towards Mars.

It is important to note that the
red arrows do not imply cometary rotation; instead, they represent
shear deformation, and illustrate the torque generated at the neck.

- Comets are thought to have formed
in the earliest days of the Solar System, and are stored in vast clouds
at its outer edges before beginning their journey inwards. It would
have been during this initial ‘building’ phase of the Solar
System that 67P/C-G got its initial shape.

Legend to Figure 39:
First impressions of the Kuiper Belt object Ultima Thule (left)
revealed a surprisingly familiar appearance to the comet that ESA's
Rosetta spacecraft explored for more than two years (right). - NASA's New Horizons flew by Ultima Thule on 1 January 2019, with subsequent images and data
suggesting that its two lobes are rather more 'squashed' like a
pancake, with respect to Comet 67P/Churyumov-Gerasimenko. Ultima Thule,
which sits beyond the orbit of Neptune in the outskirts of the Solar
System, is about 34 km long, with the two lobes measuring about 19.5
and 14.2 km across. — By comparison, Comet 67P/C-G's two lobes
measure 4.1 x 3.3 x 1.8 km and 2.6 x 2.3 x 1.8 km. The comet likely
originated from the Kuiper Belt and now orbits around the Sun on a 6.5
year journey that takes it from just beyond the orbit of Jupiter at its
most distant, to between the orbits of Earth and Mars at its closest.

- The data revealed that this object
also has a dual-lobed shape, even though somewhat flattened with
respect to Rosetta’s comet.

- “The similarities in shape
are promising, but the same stress structures don’t seem to be
apparent in Ultima Thule,” comments Christophe.

- As more detailed images are returned and analyzed, time will tell if it has experienced a similar history to 67P/C-G or not.

- “Comets
are crucial tools for learning more about the formation and evolution
of the Solar System,” says Matt Taylor, ESA’s Rosetta
Project Scientist.

- “We’ve only explored a
handful of comets with spacecraft, and 67P is by far the one
we’ve seen in most detail. Rosetta is revealing so much about
these mysterious icy visitors and with the latest result we can study
the outer edges and earliest days of the Solar System in a way
we’ve never been able to do before.”

• February 11, 2019: It is
always reassuring to catch that first familiar glimpse of home after a
great adventure, but for our space-faring satellites the return visit
is brief and of a practical nature: to use the planet’s immense
gravity to sling it onto a new trajectory. 78)

- These ‘gravity assists’
are fleeting encounters, but enough to change the spacecraft’s
speed and direction such that it can eventually enter orbit around
another world.

- This delicate view of Earth was captured in 2007 on the second of three Earth flybys made by ESA’s comet-chasing Rosetta
spacecraft on its ten year journey to Comet 67P/Churyumov-Gerasimenko.
The spacecraft also got a boost from Mars to set it on course with its
destination.

- The first ever interplanetary
gravity slingshot took place on 5 February 1974, when NASA’s
Mariner 10 flew past Venus en route to flybys of Mercury.

- The ESA-JAXA BepiColombo
mission – whose name is inherited from Giuseppe Colombo who
originally proposed to NASA the interplanetary trajectories that would
allow Mariner-10 multiple Mercury flybys by using gravity assists at
Venus – will make nine flybys of Earth, Venus and Mercury to
reach the innermost planet and eventually enter orbit about it.

- Similarly, ESA’s upcoming Solar Orbiter
mission will use Venus gravity assists to change its inclination to get
a better look at the Sun’s poles. And ESA’s Jupiter Icy Moons Explorer will first dive into the inner Solar System to use Earth, Venus and Mars to set course for the gas giant Jupiter.

- But Earth remains home to a fleet
of satellites busy performing a number of different activities from
orbit: while some are peering far away into the cosmos, our Earth
Observation missions are watching diligently over our precious planet,
taking its ‘pulse’ and helping us to better understand how
to care for it. The Sun-illuminated crescent seen around Antarctica in
this beautiful image (Figure 41) certainly evokes a feeling of fragility and reminds us of our special place in space.

• January 31, 2019:
Engineering models are an important part of spacecraft operations
– acting as faithful and realistic testbeds for all sorts of
trials and tricks too risky to attempt, first-go, on the original.
Models like this also serve as mementos of our human endeavors in
space, which are so often hard to visualize, and even harder to get
close to. 80)

- The original Rosetta probe carried
out its final maneuver at 20:50 GMT (22:50 CEST) on 29 September 2016,
setting itself down on comet 67P/Churyumov–Gerasimenko and
sending its final image from just 24 or so meters above the surface.

- While we no longer receive updates
from the plucky comet-chaser, our 'super model' – seen here at
night in its new glassed-in pavilion – reminds us every day of
what a remarkable achievement this was.

Figure 42: This life-size copy
of the world-famous Rosetta spacecraft is living out its retirement at
ESA’s European Space Operations Center in Darmstadt, Germany
(image credit: ESA/J. Mai)

• December 17, 2018: An old
friend of ESA, Comet 46P/Wirtanen, is crossing our skies this month.
— The comet nucleus is at the core of the brightest spot at the
center of the image, and the green diffuse cloud is its coma. The green
color is caused by molecules – mainly CN (cyanogen) and C2 (diatomic
carbon) – that are ionized by sunlight as the comet approaches
the Sun. A hint of the comet’s tail is visible to the upper left;
the diagonal stripes are star trails. 81)82)

- A bright
comet with a period of 5.5 years, 46P had been chosen in the 1990s as
the target of ESA’s Rosetta mission. However, a launch delay from
2003 to 2004 meant the spacecraft would not be able to rendezvous with
that comet at its closest approach to the Sun in 2013, prompting the
Rosetta team to select a new target, the now famed
67P/Churyumov­–Gerasimenko.

- Comet 46P was at perihelion, the
closest point to the Sun along its orbit, on 12 December, and kept
moving towards our planet, reaching the closest distance to Earth on 16
December.

- Astronomers across the world
– professional, student and amateur alike – have been
observing the comet recently, and will keep doing so in coming weeks as
it moves away from the Sun along its orbit.

Figure 43: This image was taken
by Wouter Van Reeven at ESA/ESAC (European Space Astronomy Center) near
Madrid, Spain, on 14 December 2018. It is a composite of 132 individual
images, each with a 10 second exposure, using a William Optics ZS 71 ED
(71 mm refractor) telescope and a Canon EOS 700D DSLR camera (ISO:
3200). The field of view spans 2.8º x 1.8º (image credit:
ESA/ESAC Astronomy Club / W. Van Reeven)

• December 12, 2018: A new
study reveals that, contrary to first impressions, Rosetta did detect
signs of an infant bow shock at the comet it explored for two years
– the first ever seen forming anywhere in the Solar System. 83)84)

- From 2014 to 2016, ESA’s Rosetta spacecraft studied Comet 67P/Churyumov-Gerasimenko
and its surroundings from near and far. It flew directly through the
‘bow shock’ several times both before and after the comet
reached its closest point to the Sun along its orbit, providing a
unique opportunity to gather in situ measurements of this intriguing
patch of space.

- As the supersonic solar wind flows
past objects in its path, such as planets or smaller bodies, it first
hits a boundary known as a bow shock. As the name suggests, this
phenomenon is somewhat like the wave that forms around the bow of a
ship as it cuts through choppy water.

Figure 44:
Artist’s impression of the infant bow shock detected by
ESA’s Rosetta spacecraft at Comet 67P/Churyumov-Gerasimenko. The
spacecraft detected signs of a forming bow shock around 50 times closer
to the comet’s nucleus than anticipated in the case of 67P. This
boundary was observed to be asymmetric, wider than the fully developed
bow shocks observed at other comets, and moving in unexpected ways. -
Rosetta detected the bow shock as the boundary changed position
responding to the upstream magnetic field flipping from one side to the
other. As a result, the spacecraft found itself alternatively outside
of the shock (left frame) and behind it (right frame). It is the first time a bow shock in such an early formation stage has been detected anywhere in the Solar System(image credit: ESA)

- Bow shocks have been found around comets, too – Halley’s comet
being a good example. Plasma phenomena vary as the medium interacts
with the surrounding environment, changing the size, shape, and nature
of structures such as bow shocks over time.

- Rosetta looked for signs of such a feature over its two-year mission, and ventured over 1500 km away from 67P’s center on the hunt for large-scale boundaries around the comet – but apparently found nothing.

- “We looked for a classical
bow shock in the kind of area we’d expect to find one, far away
from the comet’s nucleus, but didn’t find any, so we
originally reached the conclusion that Rosetta had failed to spot any
kind of shock,” says Herbert Gunell of the Royal Belgian
Institute for Space Aeronomy, Belgium, and Umeå University,
Sweden, one of the two scientists who led the study.

- “However, it seems that the
spacecraft actually did find a bow shock, but that it was in its
infancy. In a new analysis of the data, we eventually spotted it around
50 times closer to the comet’s nucleus than anticipated in the
case of 67P. It also moved in ways we didn’t expect, which is why
we initially missed it.”

- On 7 March 2015, when the comet
was over twice as far from the Sun as the Earth and heading inwards
towards our star, Rosetta data showed signs of a bow shock beginning to
form. The same indicators were present on its way back out from the
Sun, on 24 February 2016.

- This boundary was observed to be asymmetric, and wider than the fully developed bow shocks observed at other comets.

- “Such an early phase of the
development of a bow shock around a comet had never been captured
before Rosetta,” says co-lead Charlotte Goetz of the Institute
for Geophysics and Extraterrestrial Physics in Braunschweig, Germany.

- “The
infant shock we spotted in the 2015 data will have later evolved to
become a fully developed bow shock as the comet approached the Sun and
became more active – we didn't see this in the Rosetta data,
though, as the spacecraft was too close to 67P at that time to detect
the ‘adult’ shock. When Rosetta spotted it again, in 2016,
the comet was on its way back out from the Sun, so the shock we saw was
in the same state but ‘unforming’ rather than
forming.”

- Herbert, Charlotte, and colleagues
explored data from the Rosetta Plasma Consortium, a suite of
instruments comprising five different sensors to study the plasma
surrounding Comet 67P. They combined the data with a plasma model to
simulate the comet’s interactions with the solar wind and
determine the properties of the bow shock.

- The scientists found that, when
the forming bow shock washed over Rosetta, the comet’s magnetic
field became stronger and more turbulent, with bursts of highly
energetic charged particles being produced and heated in the region of
the shock itself. Beforehand, particles had been slower-moving, and the
solar wind had been generally weaker – indicating that Rosetta
had been ‘upstream’ of a bow shock.

- “These observations are the
first of a bow shock before it fully forms, and are unique in being
gathered on-location at the comet and shock itself,” says Matt
Taylor, ESA Rosetta Project Scientist. “This finding also
highlights the strength of combining multi-instrument measurements and
simulations. It may not be possible to solve a puzzle using one
dataset, but when you bring together multiple clues, as in this study,
the picture can become clearer and offer real insight into the complex
dynamics of our Solar System – and the objects in it, like
67P.”

Legend to Figure 45:
The comet is represented in grey in the left-hand frame, while the
small cyan satellite represents the simulated Rosetta spacecraft. The
simulation reconstructs the plasma conditions when Rosetta spotted an
infant bow shock in the process of ‘unforming’: this infant
bow shock can be seen as the sweeping red-yellow curve. The colors show
the proton density for the region – the number of protons found
within a cubic cm – as indicated in the bar at the top, with
red-yellow being a high and black-blue a low density. In this frame,
the Sun is on the right-hand side, meaning that the solar wind flows
from right to left.

As Rosetta circles around the comet
in this simulation, crossing the bow shock twice, the associated
patterns observed in proton and water ion abundances and energies can
be seen in the upper and central panels on the right, with the moving
cyan line representing Rosetta’s location. This demonstrates how
the major types of charged particles in the vicinity of the forming bow
shock change along with Rosetta’s simulated location, and how the
distribution of protons becomes wider in the region of the shock than
in the surrounding solar wind (shown in the two vertical stripes of red
in the top right panel). The bars to the right indicate the
differential particle flux to the instrument – the number of
particles passing through a given area per unit time and solid angle (a
measure of the instrument’s field of view) – with red being
high and black being low. The scales to the left indicate particle
energies.

The magnetic field strength around
the comet can be seen in the bottom-right panel. As the simulated
Rosetta crosses the shock, two spikes appear in the field strength,
which also correlate with the shock-related effects seen in the upper
two right-hand panels.

• October 01, 2018: On 30
September 2016, ESA’s Rosetta spacecraft came closer than ever to
the target it had studied from afar for more than two years, concluding its mission with a controlled impact onto the surface of Comet 67P/Churyumov-Gerasimenko (67P/C-G). 85)

- This image shows a portion of
67P/C-G as viewed by Rosetta on 22 September 2014, only one and a half
months after the spacecraft had made its rendezvous with the comet.
At the time, the spacecraft was 28.2 km from the comet center (around
26.2 km from the surface). Amateur astronomer Jacint Roger Perez, from
Spain, selected and processed this view by combining three images taken
in different wavelengths by the OSIRIS narrow-angle camera on Rosetta.

- Seen in the center and left of the
frame is Seth, one of the geological regions on the larger of the two
comet lobes, which declines towards the smoother Hapi region on the
comet’s ‘neck’ that connects the two lobes. The
landscape in the background reveals hints of the Babi and Aker regions,
both located on the large lobe of 67P/C-G. For a wider image of this
region in the overall context of the comet see here.

- The sharp profile in the lower
part of the image shows the Aswan cliff, a 134 m-high scarp separating
the Seth and Hapi regions. Observations performed by Rosetta not long
before the comet’s perihelion, which took place on 13 August
2015, revealed that a chunk of this cliff had collapsed – a consequence of increased activity as the comet drew closer to the Sun along its orbit.

Figure 46:
An evocative image of Rosetta’s comet to recall the end of its
trailblazing mission two years ago (image credit: ESA/Rosetta/MPS for
OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; J. Roger – CC
BY SA 4.0)

• August 6, 2018: Over its
lifetime Rosetta extensively mapped the comet’s surface, which
has since been divided into 26 geological regions
named after Ancient Egyptian deities. The entire comet has been likened
to a duck in shape, with a small ‘head’ attached to a
larger ‘body’. 86)

Figure 47: This image shows a
section of 67P/C-G as viewed by Rosetta’s high-resolution camera
OSIRIS on 10 February 2016. Amateur astronomer Stuart Atkinson, from
the UK, selected and processed this view from the OSIRIS image archive.
It is a crop of a larger image that shows a slightly wider view
of the comet’s ‘Bes’ region on body of the comet,
which takes its name from the protective deity of households, children
and mothers (image credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA – CC BY SA 4.0;
Acknowledgement: S Atkinson)

- The image shows the uneven,
shadowed surface of the comet in detail; particularly prominent just to
the right of center is an upright feature surrounded by scattered
depressions, rocky outcrops and debris.

• July 18, 2018: As
Japan’s Hayabusa-2 drew closer to its target Ryugu asteroid, a
strange new planetoid came into view – but one with a somewhat
familiar shape. This distinct ‘spinning top’ asteroid class
has been seen repeatedly in recent years, and might give a foretaste of
things to come for ESA’s proposed Hera mission. 87)

- Hayabusa2 is currently just 20 km
away from the 900-m wide asteroid. The view from its navigation camera
reveals a spinning body with an enlarged ridge of material around its
equator – a bulge suggesting Ryugu may once have been spinning
much faster.

- As ESA’s space scientist
Michael Küppers followed Hayabusa-2’s approach, he recalled
Europe’s own asteroid first encounter, just under a decade ago on
5 September 2008, when Rosetta performed a flyby of the Steins asteroid en route to its final destination, comet 67P/Churyumov-Gerasimenko.

Figure 50:
Sequential views from Japan's Hayabusa-2 mission as it approached the
Ryugu asteroid during June 2018 (image credit: JAXA, University of
Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba
Institute of Technology, Meiji University, University of Aizu and AIST)

- “At 6 km across,
Šteins (Steins) was much larger, but had a similar diamond
shape,” says Michael. “Personally I wasn’t surprised
to see this again with Ryugu, because it has turned up with many
smaller asteroids in recent years. The current thinking is this shape
is due to asteroids being set spinning rapidly, and the resulting
centrifugal force moving material away from the poles and towards the
equator. As for what causes such a spin, this probably comes down to
the so-called YORP
(Yarkovsky–O'Keefe–Radzievskii–Paddack)
effect.”

- The YORP effect, named after four
different researchers who worked on asteroids, is triggered by the
warming of asteroids by sunlight. The asteroids re-radiate this energy
as heat, which gives rise to a tiny amount of thrust. Eventually
Newton’s Third Law – ‘every action has an equal and
opposite reaction’ – exerts itself. And due to their
irregular shapes, some parts of asteroids generate more thrust than
others, leading to a turning force like wind past a windmill.

- “The resulting centrifugal
force could continue to the point that material is actually thrown out
into space,” adds Michael, “leading to the creation of the
binary or multiple asteroid systems that make up 15% of all asteroids
so far discovered. Some might also crumble apart altogether. For larger
asteroids YORP is less likely to influence shape, as their ratio
between mass and surface area is much higher.”

- Today Michael is serving as
project scientist on ESA’s Hera mission study, planned as
humankind’s first mission to a binary asteroid system if approved
at next year’s ESA Council meeting at ministerial level. His role
is to work with external scientists to come up with mission
requirements, and make early plans for operations and data analysis.

- Hera’s target is the Didymos
system, with a 780 m main body orbited by a smaller 160 m
‘Didymoon’. NASA’s DART spacecraft will impact this
smaller body in 2022 to measurably shift its orbit, ahead of
Hera’s arrival in 2026 – the two missions combining in an
audacious, full-scale planetary defence test.

• June 21, 2018: All
high-resolution images and the underpinning data from Rosetta's
pioneering mission at Comet 67P/Churyumov-Gerasimenko are now available
in ESA's archives, with the last release including the iconic images of
finding lander Philae, and Rosetta's final descent to the comet's
surface. 89)

- The Archive Image Browser also
hosts images captured by the spacecraft's Navigation Camera, while the
Planetary Science Archive contains publicly available data from all
eleven science instruments onboard Rosetta – as well as from
ESA's other Solar System exploration missions.

- The final batch of high-resolution
images from Rosetta's OSIRIS camera covers the period from late July
2016 to the mission end on 30 September 2016. It brings the total count
of images from the narrow- and wide-angle cameras to nearly 100 000
across the spacecraft's 12 year journey through space, including early
flybys of Earth, Mars and two asteroids before arriving at the comet.

- The spacecraft's trajectory around the comet
changed progressively during the final two months of the mission,
bringing it closer and closer at its nearest point along elliptical
orbits. This allowed some spectacular images to be obtained from within
just two kilometers of the surface, highlighting the contrasts in
exquisite detail between the smooth and dusty terrain, and more
consolidated, fractured comet material.

- One particularly memorable sets of images captured in this period were those of Rosetta's lander Philae following the painstaking effort
over the previous years to determine its location. With Rosetta flying
so close, challenging conditions associated with the dust and gas
escaping from the comet, along with the topography of the local
terrain, caused problems with getting the best line-of-sight view of
Philae's expected location, but the winning shot was finally captured just weeks before the mission end.

- In the mission's last hours as
Rosetta moved even closer towards the surface of the comet, it scanned
across an ancient pit and finally sent back images showing what would
become its resting place. Even after the spacecraft was silent, the
team were able to reconstruct a last image from the final telemetry packets sent back when Rosetta was within about 20 m of the surface.

• January 22, 2018: Perhaps
you live in a part of the world where you regularly experience snow
storms or even dust storms. But for many of us, the weather forms a
natural part of everyday conversation – more so when it is
somewhat extreme, like a sudden blizzard that renders transport useless
or makes you feel highly disoriented as you struggle to fix your sights
on recognizable landmarks.

- ESA’s Rosetta mission had a
similar experience, for more than two years, as it flew alongside Comet
67P/Churyumov–Gerasimenko between 2014 and 2016. It endured the
endless impacts of dust grains launched by gaseous outpourings as the
comet’s surface ices were warmed by the heat of the Sun,
evaporating into space and dragging the dust along. 90)

- The image of Figure 51 was taken two years ago, on 21 January 2016, when Rosetta was flying 79 km from the comet. At this time Rosetta was moving closer following perihelion
in the previous August, when the comet was nearer to the Sun and as
such at its most active, meaning that Rosetta had to operate from a
greater distance for safety.

- As can be
seen from the image, the comet environment was still extremely chaotic
with dust even five months later. The streaks reveal the dust grains as
they passed in front of Rosetta’s camera, captured in the 146
second exposure.

- Excessive dust in Rosetta’s
field of view presented a continual risk for navigation: the
craft’s startrackers used a star pattern recognition function to
know its orientation with respect to the Sun and Earth. On some
occasions flying much closer to the comet, and therefore through denser
regions of outflowing gas and dust, the startrackers locked on to dust
grains instead of stars, creating pointing errors and in some cases
putting the spacecraft in a temporary safe mode.

- Despite its dangers, the dust was
of high scientific interest: three of Rosetta’s instruments
studied tens of thousands of grains between them, collectively analyzing their composition, their mass, momentum and velocity, and profiling their 3D structure. Studying the smallest and the most pristine grains ejected is helping scientists to understand the building blocks of comets.

- Two years before the image was taken, 20 January 2014, Rosetta was only just waking up from 31 months of deep-space hibernation.
It arrived at its destination after 10 years in space in August 2014,
and released the lander Philae three months later. Rosetta made unique scientific observations of the comet until reaching its grand finale
on 30 September 2016 by descending to the comet’s surface. By the
end of the mission, more than a hundred thousand images had been taken
by the high-resolution OSIRIS camera (including the one shown here) and
the navigation camera, the majority of which are available to browse in
the Archive Image Browser.

Figure 51:
This stormy day image was taken with the OSIRIS narrow angle camera two
years ago, on 21 January 2016, when Rosetta was flying 79 km from the
comet (image credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

• October 26, 2017: Last year,
a fountain of dust was spotted streaming from Rosetta's comet,
prompting the question: how was it powered? Scientists now suggest the
outburst was driven from inside the comet, perhaps released from
ancient gas vents or pockets of hidden ice. 91)

- The plume was seen by ESA's
Rosetta spacecraft on 3 July 2016, just a few months before the end of
the mission and as Comet 67P/Churyumov–Gerasimenko was heading
away from the Sun at a distance of almost 500 million km.

- "We saw a bright plume of dust
blowing away from the surface like a fountain," explains Jessica
Agarwal of the Max Planck Institute for Solar System Research in
Göttingen, Germany, and lead author of the new paper. "It lasted
for roughly an hour, producing around 18 kg of dust every second."

- Alongside a steep increase in the
number of dust particles flowing from the comet, Rosetta also detected
tiny grains of water-ice. The images showed the location of the
outburst: a 10 m-high wall around a circular dip in the surface.

- Previous plumes, collapsing cliffs
and similar features have been seen on the comet, but spotting this one
was especially fortunate: as well as imaging the location in detail,
Rosetta also sampled the ejected material itself.

- "This plume was really special. We
have great data from five different instruments on how the surface
changed and on the ejected material because Rosetta was, by chance,
flying through the plume and looking at the right part of the surface
when it happened," adds Jessica. "Rosetta hasn't provided such detailed
and comprehensive coverage of an event like this before."

- Initially, scientists thought that
the plume might have been surface ice evaporating in the sunlight.
However, Rosetta's measurements showed there had to be something more
energetic going on to fling that amount of dust into space. "Energy
must have been released from beneath the surface to power it," says
Jessica. "There are evidently processes in comets that we do not yet
fully understand."

- How such energy was released
remains unclear. Perhaps it was pressurized gas bubbles rising through
underground cavities and bursting free via ancient vents, or stores of
ice reacting violently when exposed to sunlight.

- "One of Rosetta's major goals was
to understand how a comet works. For example, how does its gaseous
envelope form and change over time?" says Matt Taylor, ESA's Rosetta
Project Scientist. -"Outbursts are interesting because of this, but we
weren't able to predict when or where they would occur – we had
to be lucky to capture them. Having full, multi-instrument coverage of
an outburst like this and its effect on the surface is really valuable
for revealing how these events are driven. Rosetta scientists are now
combining measurements from the comet with computer simulations and
laboratory work to find out what drives such plumes on comets." 92)

• September 28, 2017:
Scientists analyzing the final telemetry sent by Rosetta immediately
before it shut down on the surface of the comet last year have
reconstructed one last image of its touchdown site. After more than 12
years in space, and two years following Comet
67P/Churyumov–Gerasimenko as they orbited the Sun,
Rosetta’s historic mission concluded on 30 September with the
spacecraft descending onto the comet in a region hosting several
ancient pits. It returned a wealth of detailed images and scientific
data on the comet’s gas, dust and plasma as it drew closer to the
surface. 93)

Figure 54:
A final image from Rosetta, shortly before it made a controlled impact
onto Comet 67P/Churyumov–Gerasimenko on 30 September 2016, was
reconstructed from residual telemetry. The image has a scale of 2
mm/pixel and measures about 1 m across (image credit: ESA/Rosetta/MPS
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

- But there was one last surprise in
store for the camera team, who managed to reconstruct the final
telemetry packets into a sharp image. “The last complete image
transmitted from Rosetta was the one that we saw arriving back on Earth
in one piece moments before the touchdown at Sais,” says Holger
Sierks, principal investigator for the OSIRIS camera at the Max Planck
Institute for Solar System Research in Göttingen, Germany. -
“Later, we found a few telemetry packets on our server and
thought, wow, that could be another image.”

- During operations, images were
split into telemetry packets aboard Rosetta before they were
transmitted to Earth. In the case of the last images taken before
touchdown, the image data, corresponding to 23,048 bytes per image,
were split into six packets.

- For the very last image the
transmission was interrupted after three full packets were received,
with 12,228 bytes received in total, or just over half of a complete
image. This was not recognized as an image by the automatic processing
software, but the engineers in Göttingen could make sense of these
data fragments to reconstruct the image.

Legend to Figure 55:
In order to give a feeling of scale, this artist impression of the
Rosetta spacecraft is superimposed on an OSIRIS wide-angle camera image
of the region in which it landed on 30 September 2016. Also marked on
the image are the approximate locations of the final two images taken
by the spacecraft from around 20 m altitude. The cross indicates the
estimated center of touchdown of Rosetta. -The background image
measures about 55 m across, while the final images are about 1 m
across. For comparison, Rosetta measures 32 m from tip to tip, and its
solar panels are a little more than 2 m high each. — Note that
the positioning of the spacecraft on the image is not an accurate
representation of the actual landing.

Legend to Figure 56:
Annotated image indicating the approximate locations of some of
Rosetta’s final images. Note that due to differences in timing
and viewing geometry between consecutive images in this graphic, the
illumination and shadows vary.

- Top left: a global view of Comet
67P/Churyumov–Gerasimenko shows the area in which Rosetta touched
down in the Ma’at region on the smaller of the two comet lobes.
This image was taken by the OSIRIS narrow-angle camera on 5 August 2014
from a distance of 123 km.

- Top right: an image taken by the
OSIRIS narrow-angle camera from an altitude of 5.7 km, during
Rosetta’s descent on 30 September 2016. The image scale is about
11 cm/pixel and the image measures about 225 m across. The final
touchdown point, named Sais, is seen in the bottom right of the image
and is located within a shallow, ancient pit. Exposed, dust-free
terrain is seen in the pit walls and cliff edges. Note the image is
rotated 180º with respect to the global context image at top
right.

- Middle: an OSIRIS wide-angle
camera image taken from an altitude of about 331 m during
Rosetta’s descent. The image scale is about 33 mm/pixel and the
image measures about 55 m across. The image shows a mix of coarse and
fine-grained material.

- Bottom right: the penultimate
image, which was the last complete image taken and returned by Rosetta
during its descent, from an altitude of 24.7±1.5 m.

- Bottom left: the final image,
reconstructed after Rosetta’s landing, was taken at an altitude
of 19.5±1.5 m. The image has a scale of 2 mm/pixel and measures
about 1 m across.

• June 8, 2017: The
challenging detection, by ESA's Rosetta mission, of several isotopes of
the noble gas xenon at Comet 67P/Churyumov-Gerasimenko has established
the first quantitative link between comets and the atmosphere of Earth.
The blend of xenon found at the comet closely resembles U-xenon, the
primordial mixture that scientists believe was brought to Earth during
the early stages of Solar System formation. These measurements suggest
that comets contributed about one fifth the amount of xenon in Earth's
ancient atmosphere. 94)

- Xenon – a colorless,
odorless gas which makes up less than one billionth of the volume of
Earth's atmosphere – might hold the key to answer a long-standing
question about comets: did they contribute to the delivery of material
to our planet when the Solar System was taking shape, some 4.6 billion
years ago? And if so, by how much?

- The noble gas xenon is formed in a
variety of stellar processes, from the late phases of low- and
intermediate-mass stars to supernova explosions and even neutron star
mergers. Each of these phenomena gives rise to different isotopes of
the element. As a noble gas, xenon does not interact with other
chemical species, and is therefore an important tracer of the material
from which the Sun and planets originated, which in turns derives from
earlier generations of stars.

- "Xenon is the heaviest stable
noble gas and perhaps the most important because of its many isotopes
that originate in different stellar processes: each one provides an
additional piece of information about our cosmic origins," says Bernard
Marty from CRPG-CNRS and Université de Lorraine, France. Bernard
is the lead author of a paper reporting Rosetta's discovery of xenon at
Comet 67P/C-G, which is published today in Science. 95)

- It is because of this special
'fingerprint' that scientists have been using xenon to investigate the
composition of the early Solar System, which provides important clues
to constrain its formation. Over the past decades, they sampled the
relative abundances of its various isotopes at different locations: in
the atmosphere of Earth and Mars, in meteorites deriving from
asteroids, at Jupiter, and in the solar wind – the flow of
charged particles streaming from the Sun.

Figure 57:
The blend of isotopes of the noble gas xenon detected by ESA's Rosetta
mission at Comet 67P/Churyumov-Gerasimenko, compared with the mixture
of xenon measured in other regions of the Solar System. All abundances
are normalized with respect to the abundance observed in the solar
wind, the flow of charged particles streaming from the Sun (shown as a
yellow line), image credit: Data from B. Marty et al., 2017 and
references therein

The blend of xenon measured
in chondrite meteorites that came from asteroids (grey line) is quite
similar to that found in the solar wind, while the one present in the
atmosphere of our planet (blue line) contains a higher abundance of
heavier isotopes with respect to the lighter ones.

However, the latter is a
result of lighter elements escaping more easily from Earth's
gravitational pull and being lost to space in greater amounts. By
correcting the atmospheric composition of xenon for this runaway
effect, scientists in the 1970s calculated the composition of the
primordial mixture of this noble gas, known as U-xenon, that was once
present on Earth. This U-xenon contained a similar mix of light
isotopes to that of asteroids and the solar wind, but included
significantly smaller amounts of the heavier isotopes.

Observations from Rosetta
revealed that the blend of xenon at Comet 67P/C-G (black data points
and line) contains larger amounts of light isotopes than heavy ones,
and so it is quite different from the average mixture found in the
Solar System. A comparison with the on-board calibration sample (blue
data points) confirmed that the xenon detected at the comet is also
different from the current mix in the Earth's atmosphere.

By contrast, the composition
of xenon detected at the comet seems to be closer to the composition
that scientists think was present in the early atmosphere of Earth.

Rosetta's measurements of
xenon at Comet 67P/C-G suggest that comets contributed about one fifth
the amount of xenon in Earth's ancient atmosphere. They also indicate
that the protosolar cloud from which the Sun, planets, and small bodies
were born was a rather inhomogeneous place in terms of its chemical
composition.

- The blend of xenon present in the
atmosphere of our planet contains a higher abundance of heavier
isotopes with respect to the lighter ones; however, this is a result of
lighter elements escaping more easily from Earth's gravitational pull
and being lost to space in greater amounts. By correcting the
atmospheric composition of xenon for this runaway effect, scientists in
the 1970s calculated the composition of the primordial mixture of this
noble gas, known as U-xenon, that was once present on Earth.

- This U-xenon contained a similar
mix of light isotopes to that of asteroids and the solar wind, but
included significantly smaller amounts of the heavier isotopes.

- "For these reasons, we have long
suspected that xenon in the early atmosphere of Earth could have a
different origin from the average blend of this noble gas found in the
Solar System," says Bernard.

- One of the explanations is that
Solar System xenon derives directly from the protosolar cloud, a mass
of gas and dust that gave rise to the Sun and planets, while the xenon
found in the Earth's atmosphere was delivered at a later stage by
comets, which in turn might have formed from a different mix of
material.

- With ESA's Rosetta mission
visiting Comet 67P/Churyumov-Gerasimenko, an icy fossil of the early
Solar System, scientists could finally gather the long-sought data to
test this hypothesis.

- "Searching for xenon at the comet
was one of the most crucial and challenging measurements we performed
with Rosetta," says Kathrin Altwegg from the University of Bern,
Switzerland, principal investigator of ROSINA, the Rosetta Orbiter
Spectrometer for Ion and Neutral Analysis, which was used for this
study.

- Xenon is very diffuse in the
comet's thin atmosphere, so the navigation team had to fly Rosetta very
close – 5 km to 8 km from the surface of the nucleus – for
a period of three weeks so that ROSINA could obtain a significant
detection of all the relevant isotopes.

- Flying so close to the comet was
extremely challenging because of the large amount of dust that was
lifting off the surface at the time, which could confuse the star
trackers that were used to orient the spacecraft.

- Eventually, the Rosetta team
decided to perform this operation in the second half of May 2016. This
period was chosen as a compromise so that enough time would have passed
after the comet's perihelion, in August 2015, for the dust activity to
be less intense, but not too much for the atmosphere to be excessively
thin and the presence of xenon hard to detect.

- As a result of the observations,
ROSINA identified seven isotopes of xenon, as well as several isotopes
of another noble gas, krypton; these brought to three the inventory of
noble gases found at Rosetta's comet, following the discovery of argon
from measurements performed in late 2014.

- "These measurements required a
long stretch of dedicated time solely for ROSINA, and it would have
been very disappointing if we hadn't detected xenon at Comet 67P/C-G,
so I'm really glad that we succeeded in detecting so many isotopes,"
adds Kathrin.

- Further analysis of the data
revealed that the blend of xenon at Comet 67P/C-G, which contains
larger amounts of light isotopes than heavy ones, is quite different
from the average mixture found in the Solar System. A comparison with
the on-board calibration sample confirmed that the xenon detected at
the comet is also different from the current mix in the Earth's
atmosphere.

- By contrast,
the composition of xenon detected at the comet seems to be closer to
the composition that scientists think was present in the early
atmosphere of Earth.

- "This is a very exciting result
because it is the first discovery of a candidate for the hypothesized
U-xenon," explains Bernard.

- "There are some discrepancies
between the two compositions, which indicate that the primordial xenon
delivered to our planet could derive from a combination of impacting
comets and asteroids."

- In particular, Bernard and his
colleagues were able to establish the first quantitative link between
comets and our planet's gaseous shroud: based on the Rosetta
measurements at Comet 67P/C-G, 22 percent of the xenon once present in
Earth's atmosphere could originate from comets – the rest being
delivered by asteroids.

- This result is not in
contradiction with the isotopic measurements of water at Rosetta's
comet, which were significantly different to that found on Earth. In
fact, given the trace amounts of xenon in Earth's atmosphere and the
much larger amount of water in the oceans, comets could have
contributed to atmospheric xenon without having a significant impact on
the composition of water in the oceans.

- The contribution inferred from the
xenon measurements, instead, agrees with the possibility that comets
have been significant carriers of pre-biotic material – such as
phosphorus and the amino acid glycine, which were also detected by
Rosetta at the comet – that was crucial to the emergence of life
on Earth.

- Finally, the difference between
the blend of xenon found at the comet – which was incorporated in
the nucleus at the time of its formation – and the xenon observed
across the Solar System indicates that the protosolar cloud from which
the Sun, planets, and small bodies were born was a rather inhomogeneous
place in terms of its chemical composition.

- "This conclusion is in accord with
previous measurements performed by Rosetta, including the unexpected
detections of molecular oxygen (O2) and di-sulphur (S2), and the high deuterium-to-hydrogen ratio observed in the comet water," adds Kathrin.

- Additional evidence for the
inhomogeneous nature of the protosolar cloud came also from anther
study based on ROSINA observations, published in May in Astronomy &
Astrophysics, which revealed that the mixture of silicon isotopes seen
at the comet is different from what is measured elsewhere in the Solar
System.

• March 21, 2017: Rosetta
scientists have made the first compelling link between an outburst of
dust and gas and the collapse of a prominent cliff, which also exposed
the pristine, icy interior of the comet. 97)
Growing fractures, collapsing cliffs, rolling boulders and moving
material burying some features on the comet’s surface while
exhuming others are among the remarkable changes documented during
Rosetta’s mission. 98)99)100)

Figure 58: A 3D view of the
Aswan cliff before and after part of it collapsed. The cliff was
originally observed to have a 70 m-long, 1 m-wide fracture separating
an overhanging block 12 m across from the main plateau. After the
collapse, bright, pristine material is observed in the cliff wall, with
new debris at the foot of the cliff (image credit: ESA/Rosetta/MPS for
OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; F. Scholten & F.
Preusker)

- Sudden and short-lived outbursts were observed frequently
during Rosetta’s two-year mission at Comet
67P/Churyumov–Gerasimenko. Although their exact trigger has been
much debated, the outbursts seem to point back to the collapse of weak,
eroded surfaces, with the sudden exposure and heating of volatile
material likely playing a role.

- In a study published today in
Nature Astronomy, scientists make the first definitive link between an
outburst and a crumbling cliff face, which is helping us to understand
the driving forces behind such events. 101)

Legend to Figure 59: Left:
images of the 70 m-long, 1 m-wide fracture at the top of the 134 m-high
Aswan cliff in the Seth region of Comet 67P/Churyumov–Gerasimenko
(marked with arrow). The last image of the fracture still present was
taken on 4 July 2015 (not shown here). Center: a broad plume of
dust is imaged by Rosetta’s navigation camera on 10 July 2015,
which can be traced back to an area on the comet that encompasses the
Seth region (the Aswan cliff is included within the marked rectangle). Right:
two example images taken after the cliff collapse, showing the exposed
material in the cliff face (top) and the new outline of the cliff top
(bottom).

- The first close images of the
comet taken in September 2014 revealed a 70 m-long, 1 m-wide fracture
on the prominent cliff-edge subsequently named Aswan, in the Seth
region of the comet, on its large lobe.

- Over the course of the following
year as the comet drew ever closer to the Sun along its orbit, the rate
at which its buried ices turned to vapor and dragged dust out into
space increased along the way. Sporadic and brief, high-speed releases
of dust and gas punctuated this background activity with outbursts.

- One such
outburst was captured by Rosetta’s navigation camera on 10 July
2015, which could be traced back to a portion of the comet’s
surface that encompassed the Seth region.

Legend to Figure 60:
Sequence of images showing different views of the Aswan cliff collapse
on Comet 67P/Churyumov–Gerasimenko. The first image shows the
fracture long before it gave away on 10 July 2015. Images taken on 15
July and 26 December show the bright, pristine material exposed in the
cliff collapse, which is thought to have occurred on 10 July. Although
not obvious from these images, the brightness had faded by about 50% by
the 26 December image, showing that much of the exposed water-ice had
already sublimated by that time. The images from 2016 show different
views of the new cliff top. By August 2016, much of the cliff face had
returned to the average brightness of the comet. — Arrows are
used to mark the fracture and the exposed water-ice, and to delineate
the new cliff top outline.

- The next time the Aswan cliff was
observed, five days later, a bright and sharp edge was spotted where
the previously identified fracture had been, along with many new
meter-sized boulders at the foot of the 134 m-high cliff. “The
last time we saw the fracture intact was on 4 July, and in the absence
of any other outburst events recorded in the following ten-day period,
this is the most compelling evidence that we have that the observed
outburst was directly linked to the collapse of the cliff,” says
Maurizio Pajola, the study leader.

- The event also provided a unique
opportunity to study how the pristine water-ice otherwise buried tens
of meters inside the comet evolved as the exposed material turned to
vapor over the following months (Figure 61).

Figure 61:
Comet cliff collapse in 3D. Anaglyph images of the Aswan cliff showing
the overhang before (left) and after (right) it collapsed. The anaglyph
images were prepared for evaluating the volume of overhang that
detached in July 2015. Note the orientation between the two images is
different (image credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Pajola)

- Indeed, after the event, the
exposed cliff face was calculated to be at least six times brighter
than the overall average surface of the comet nucleus. By 26 December
2015 the brightness had faded by half, suggesting much of the water-ice
had already vaporised by that time. — And by 6 August 2016, most
of the new cliff face had faded back to the average, with only one
large, brighter block remaining.

Figure 62: Fallen cliff debris.
Color-coded plot showing the number and size distribution of boulders
at the bottom of the Aswan cliff in the Seth region of Comet
67P/Churyumov–Gerasimenko before and after a large section
collapsed on 10 July 2015. Significantly more smaller boulders are
identified than larger pieces of debris (image credit: ESA/Rosetta/MPS
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Pajola et al.
(2017)

- In addition, the team had a clear
‘before and after’ look at how the crumbling material
settled at the foot of the cliff. By counting the number of new
boulders seen after its collapse, the team estimated that 99% of the
fallen debris was distributed at the bottom of the cliff, while 1% was
lost to space.

- This corresponds to around 10,000
tons of removed cliff material, with at least 100 tons that did not
make it to the ground, consistent with estimates made for the volume of
dust in the observed plume. Furthermore, the size range of the new
debris, between 3 m and 10 m, is consistent with the distributions
observed at the foot of several other cliffs identified on the comet.

- “We see a similar trend at
the foot of other cliffs that we have not been so fortunate to have
before and after images, so this is an important validation of cliff
collapse as a producer of these debris fields,” says Maurizio.

- But what actually led to the cliff suddenly collapsing at this particular moment?

Legend to Figure 63:
Much of the comet’s regular activity can be linked back to the
steady erosion of cliff walls that are initially fractured by thermal
or mechanical erosion. These fractures propagate into the underlying
mixture of ice and dust. As the ices sublimate, the gases escape
through the fractures, acting a bit like nozzles to focus the gas flows
and picking up dust a long the way to create the distinct collimated
jets observed in Rosetta’s images. Continued cracking, heating
and sublimation eventually leads to sudden collapse of the cliff wall
– the likely source of more-transient outburst events. At the
same time, the debris that falls to the foot of the cliff also exposes
previously hidden material, contributing to the observed outflow.

- An earlier study
suggested that both rapid daily changes in heating or longer-term
seasonal changes can create thermal stresses that lead to fracturing
and subsequent exposure of volatile materials, triggering a rapid
outburst that can cause the weakened cliff to collapse.

- Even though the Aswan cliff region
had been experiencing large temperature changes in the months before
the collapse, interestingly, the collapse occurred at local night,
ruling out a sudden extreme temperature change as the immediate
trigger.

- Instead, both daily and seasonal
temperature variations may have propagated fractures deeper into the
subsurface than previously considered, predisposing it to the
subsequent collapse.

- “If the fractures permeated
volatile-rich layers, heat could have been transferred to these deeper
layers, causing a loss of deeper ice,” explains Maurizio.
“The gas released by the vaporising material could further widen
the fractures, leading to a cumulative effect that eventually led to
the cliff collapse. Thanks to this particular event at Aswan, we think
that the cumulative effect led by strong thermal gradients could be one
of the most important weakening factors of the cliff structure.”

- “Rosetta’s images
already suggested that cliff collapses are important in shaping
cometary surfaces, but this particular event has provided the missing
‘before–after’ link between such a collapse, the
debris seen at the foot of the cliff, and the associated dust plume,
supporting a general mechanism where comet outbursts can indeed be
generated by collapsing material,” says Matt Taylor, ESA’s
Rosetta project scientist.

•
December 15, 2016: ESA’s Rosetta completed its incredible mission
on 30 September, collecting unprecedented images and data right until
the moment of contact with the comet's surface. Rosetta’s signal
disappeared from screens at ESA’s mission control at 11:19:37
GMT, confirming that the spacecraft had arrived on the surface of Comet
67P/Churyumov–Gerasimenko and switched off some 40 minutes
earlier and 720 million kilometers from Earth. 102)

- One of the final pieces of
information received from Rosetta was sent by its navigation star
trackers: a report of a ‘large object’ in the field of view
– the comet horizon.

Figure 64:
Imaging ‘footprints’ of Rosetta’s OSIRIS camera
during the descent to the comet’s surface. A primary focus was
the pit named Deir el-Medina, as indicated by the number of footprints
indicated in blue. The trail of orange and red squares reflect the
change in pointing of the camera towards the impact site, subsequently
named Sais. The final image was acquired at about 20 m above the
surface, and the touchdown point was only 33 m from the center of the
predicted landing ellipse (image credit: ESA/Rosetta/MPS for OSIRIS
Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

- Reconstruction of the final
descent showed that the spacecraft gently struck the surface only 33 m
from the target point. The accuracy once again highlighted the
excellent work of the flight dynamics specialists who supported the
entire mission. The spot, just inside an ancient pit in the Ma’at
region on the comet’s ‘head’, was named Sais, after a
town where the Rosetta Stone was originally located.

- Numerous images were taken of the
neighboring pit, capturing incredible details of its layered walls that
will be used to help decipher the comet’s geological history.

- The final image was acquired about
20 m above the impact point. In addition, a number of Rosetta’s
dust, gas and plasma analysis instruments collected data.

- The pressure of the gas outflow
from the comet was seen to rise as the surface neared. Scans revealed
temperatures between about –190ºC and –110ºC down
to a few centimeters below the surface. The variation was most likely
due to shadows and local topography as Rosetta flew across the surface.

- A last measurement of water vapor
emission was made on 27 September, estimating the comet was emitting
the equivalent of two tablespoons of water per second. During its most
active period in August 2015, estimates were in the region of two
bathtubs’ worth of water every second.

Figure 65:
Rosetta's last image of Comet 67P/Churyumov-Gerasimenko, taken with the
OSIRIS wide-angle camera shortly before impact, at an estimated
altitude of about 20 m above the surface (image credit: ESA/Rosetta/MPS
for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Legend to Figure 65:
The initially reported 51 m was based on the predicted impact time. Now
that this has been confirmed, and following additional information and
timeline reconstruction, the estimated distance is now thought to be
around 20 m, and analysis is ongoing. The image scale is about 2
mm/pixel and the image measures about 96 cm across.

Figure 66:
Comet landing sites in context: Rosetta’s planned impact point in
Ma’at shown in context with Philae’s first and final
touchdown sites. All three sites are on the smaller of Comet
67P/Churyumov–Gerasimenko’s two lobes (image credit: CIVA:
ESA/Rosetta/Philae/CIVA; NAVCAM: ESA/Rosetta/NAVCAM – CC BY-SA
IGO 3.0; OSIRIS: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; ROLIS:
ESA/Rosetta/Philae/ROLIS/DLR) 103)

Legend to Figure 66:
The insets show close-up details of the three sites. Philae’s
first touchdown in Agilkia was captured by the lander’s descent
camera ROLIS; the image shown here was taken from a height of just 9 m
above the surface on 12 November 2014, and has a resolution of 0.95
cm/pixel. The view at Philae’s final touchdown site, known as
Abydos, was taken by the lander’s CIVA camera on 13 November
2014; the image shown here is a two-image mosaic, and includes one of
the lander’s feet.

- The first
indications from spectral readings show there to be no significant
differences in surface composition at the high resolutions obtained all
the way down, and there was no obvious indication of small icy patches
near the landing site. The measurements also suggest an increase in
very small dust grains – possibly around a millionth of a
millimeter – close to the surface.

- The last observation of the gas
coma surrounding the comet was made the day before the final descent.
Carbon dioxide was still being outgassed, at a greater distance from
the Sun than when the comet was approaching it. Stable solar wind
conditions reigned during the final measurements of the solar wind and
interplanetary magnetic field, providing ‘quiet’ background
values that will be important for calibration. Decreasing comet plasma
densities were observed from about 2 km above the surface, with no
obvious detection of local outgassing from the Ma’at pits.

- Magnetic field measurements down
to an estimated 11 m above the surface confirmed the previous
observations of the comet as a non-magnetic body.

- No large dust particles were
collected during the descent, in itself an interesting result. First
impressions are that the observed water vapor production was too low to
lift dust grains above a detectable size from the surface.

- “It’s great to have
these first insights from Rosetta’s last set of data,” says
Matt Taylor, ESA’s Rosetta Project Scientist. “Operations
have been completed for over two months now, and the instrument teams
are very much focused on analyzing their huge datasets collected during
Rosetta’s two-plus years at the comet.

• November 17, 2016: As
Rosetta's comet approached its most active period last year, the
spacecraft spotted carbon dioxide ice – never before seen on a
comet – followed by the emergence of two unusually large patches
of water ice.104)105)106)

- The carbon dioxide ice layer
covered an area comparable to the size of a football pitch, while the
two water ice patches were each larger than an Olympic swimming pool
and much larger than any signs of water ice previously spotted at the
comet.- The three icy layers were all found in the same region, on the
comet's southern hemisphere.

- A combination of the complex shape
of the comet, its elongated path around the Sun and the substantial
tilt of its spin, seasons are spread unequally between the two
hemispheres of the double-lobed Comet 67P/Churyumov-Gerasimenko.

- When Rosetta arrived in August
2014, the northern hemisphere was still undergoing its 5.5 year summer,
while the southern hemisphere was in winter and much of it was shrouded
in darkness. — However, shortly before the comet's closest
approach to the Sun in August 2015, the seasons changed and the
southern hemisphere experienced a brief but intense summer, exposing
this region to sunlight again.

- In the first half of 2015, as the
comet steadily became more active, Rosetta observed water vapour and
other gases pouring out of the nucleus, lifting its dusty cover and
revealing some of the comet's icy secrets. In particular, on two
occasions in late March 2015, Rosetta's visible, infrared and thermal
imaging spectrometer, VIRTIS, found a very large patch of carbon
dioxide ice in the Anhur region, in the comet's southern hemisphere.

- This is the first detection of
solid carbon dioxide on any comet, although it is not uncommon in the
Solar System – it is abundant in the polar caps of Mars, for
example. "We know comets contain carbon dioxide, which is one of the
most abundant species in cometary atmospheres after water, but it's
extremely difficult to observe it in solid form on the surface,"
explains Gianrico Filacchione from Italy's INAF-IAPS Istituto di
Astrofisica e Planetologia Spaziali, who led the study.

- In the comet environment, carbon
dioxide freezes at -193°C, much below the temperature where water
turns into ice. Above this temperature, it changes directly from a
solid to a gas, hampering its detection in ice form on the surface. By
contrast, water ice has been found at various comets, and Rosetta
detected plenty of small patches on several regions. "We hoped to find
signs of carbon dioxide ice and had been looking for it for quite a
while, but it was definitely a surprise when we finally detected its
unmistakable signature," adds Gianrico.

- The patch, consisting of a few
percent of carbon dioxide ice combined with a darker blend of dust and
organic material, was observed on two consecutive days in March. This
was a lucky catch: when the team looked at that region again around
three weeks later, it was gone.

- Assuming
that all of the ice had turned into gas, the scientists estimated that
the 80 m x 60 m patch contained about 57 kg of carbon dioxide,
corresponding to a 9 cm-thick layer. Its presence on the surface is
likely an isolated rare case, with the majority of carbon dioxide ice
being confined to deeper layers of the nucleus.

- Gianrico and his collaborators
believe the icy patch dates back a few years, when the comet was still
in the cold reaches of the outer Solar System and the southern
hemisphere was experiencing its long winter. At that time, some of the
carbon dioxide still outgassing from the interior of the nucleus
condensed on the surface, where it remained frozen for a very long
while, and vaporised only as the local temperature finally rose again
in April 2015.

- This reveals a seasonal cycle of
carbon dioxide ice, which unfolds over the comet's 6.5 year orbit, as
opposed to the daily cycle of water ice, also spotted by VIRTIS shortly
after Rosetta's arrival.

- Interestingly, shortly after the
carbon dioxide ice had disappeared, Rosetta's OSIRIS narrow-angle
camera detected two unusually large patches of water ice in the same
area, between the southern regions of Anhur and Bes (Figure 68).
"We had already seen many meter-sized patches of exposed water ice in
various regions of the comet, but the new detections are much larger,
spanning some 30 m x 40 m each, and they persisted for about 10 days
before they completely disappeared," says Sonia Fornasier from
LESIA–Observatoire de Paris and Université Paris Diderot,
France, lead scientist of the study focusing on seasonal and daily
surface color variations.

- These
ice-rich areas appear as very bright portions of the comet surface
reflecting light that is bluer in color compared with the redder
surroundings. Scientists have experimented with mixtures of dust and
water ice to show that, as the concentration of ice in them increases,
the reflected light becomes gradually bluer in color, until reaching a
point where equal amounts of light are reflected in all colors.

- The two newly detected patches
contain 20–30% of water ice mixed with darker material, forming a
layer up to 30 cm thick of solid ice. One of them was likely lurking
underneath the carbon dioxide ice sheet revealed by VIRTIS about a
month before.

• September 30, 2016:
ESA’s historic Rosetta mission has concluded as planned, with the
controlled impact onto the comet it had been investigating for more
than two years. Confirmation of the end of the mission arrived at
ESA's control center in Darmstadt, Germany at 11:19 UTC (13:19 CEST)
with the loss of Rosetta's signal upon impact into
67P/Churyumov-Gerasimenko, 718 million km from Earth. Controlled
hard-landing has become a common way to end the missions of planetary
probes. But while most have been very high-velocity impacts,
Rosetta’s touchdown was made at a sedate walking pace of 2 km/h. 107)108)109)

- Rosetta
carried out its final maneuver last night at 20:50 UTC (22:50 CEST),
setting it on a collision course with the comet from an altitude of
about 19 km. Rosetta had targeted a region on the small lobe of Comet
67P/Churyumov–Gerasimenko, close to a region of active pits in
the Ma'at region.

- The descent gave Rosetta the
opportunity to study the comet's gas, dust and plasma environment very
close to its surface, as well as take very high-resolution images. -
Pits are of particular interest because they play an important role in
the comet's activity. They also provide a unique window into its
internal building blocks.

- The information collected on the
descent to this fascinating region was returned to Earth before the
impact. It is now no longer possible to communicate with the
spacecraft.

- "Rosetta has entered the history
books once again," says Johann-Dietrich Wörner, ESA's Director
General. "Today we celebrate the success of a game-changing mission,
one that has surpassed all our dreams and expectations, and one that
continues ESA's legacy of 'firsts' at comets."

- "Thanks to a huge international,
decades-long endeavor, we have achieved our mission to take a
world-class science laboratory to a comet to study its evolution over
time, something that no other comet-chasing mission has attempted,"
notes Alvaro Giménez, ESA's Director of Science. "Rosetta was on
the drawing board even before ESA's first deep-space mission, Giotto,
had taken the first image of a comet nucleus as it flew past Halley in
1986. "The mission has spanned entire careers, and the data returned
will keep generations of scientist busy for decades to come."

- "As well as being a scientific and
technical triumph, the amazing journey of Rosetta and its lander Philae
also captured the world's imagination, engaging new audiences far
beyond the science community. It has been exciting to have everyone
along for the ride," adds Mark McCaughrean, ESA's senior science
advisor.

- Many surprising discoveries have
already been made during the mission, not least the curious shape of
the comet that became apparent during Rosetta's approach in July and
August 2014. Scientists now believe that the comet's two lobes formed
independently, joining in a low-speed collision in the early days of
the Solar System. Long-term monitoring has also shown just how
important the comet's shape is in influencing its seasons, in moving
dust across its surface, and in explaining the variations measured in
the density and composition of the coma, the comet's
‘atmosphere'.

- Some of the most unexpected and
important results are linked to the gases streaming from the comet's
nucleus, including the discovery of molecular oxygen and nitrogen, and
water with a different ‘flavor' to that in Earth's oceans.-
Together, these results point to the comet being born in a very cold
region of the protoplanetary nebula when the Solar System was still
forming more than 4.5 billion years ago.

- While it seems that the impact of
comets like Rosetta's may not have delivered as much of Earth's water
as previously thought, another much anticipated question was whether
they could have brought ingredients regarded as crucial for the origin
of life. - Rosetta did not disappoint, detecting the amino acid
glycine, which is commonly found in proteins, and phosphorus, a key
component of DNA and cell membranes. Numerous organic compounds were
also detected ­by Rosetta from orbit, and also by Philae in situ on
the surface.

- Overall, the results delivered by
Rosetta so far paint comets as ancient leftovers of early Solar System
formation, rather than fragments of collisions between larger bodies
later on, giving an unparalleled insight into what the building blocks
of the planets may have looked like 4.6 billion years ago.

- "Just as the Rosetta Stone after
which this mission was named was pivotal in understanding ancient
language and history, the vast treasure trove of Rosetta spacecraft
data is changing our view on how comets and the Solar System formed,"
says project scientist Matt Taylor. "Inevitably, we now have new
mysteries to solve. The comet hasn't given up all of its secrets yet,
and there are sure to be many surprises hidden in this incredible
archive. So don't go anywhere yet – we're only just beginning."

Figure 69:
Rosetta's OSIRIS narrow-angle camera captured this image of Comet
67P/Churyumov-Gerasimenko at 06:53 UTC from an altitude of about 8.9 km
during the spacecraft's final descent on 30 September. The image scale
is about 17 cm/pixel and the image measures about 350 m across (image
credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Figure 70:
Headed for the abyss? This photo was made from 1.2 km high just a few
minutes before impact. The scene measures just 33 m wide (image credit:
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

• September 27, 2016: Over the
past two years, Rosetta has kept a close eye on many properties of
Comet 67P/Churyumov-Gerasimenko, tracking how these changed along the
comet's orbit. A very crucial aspect concerns how much water vapor a
comet releases into space, and how the water production rate varies at
different distances from the Sun. For the first time, Rosetta enabled
scientists to monitor this quantity and its evolution in situ over two
years. 110)

- In a new study led by Kenneth C.
Hansen of the University of Michigan, Ann Arbor, MI, USA, measurements
of water production rate based on data from ROSINA, the Rosetta Orbiter
Spectrometer for Ion and Neutral Analysis, are compared with water
measurements from other Rosetta instruments. 111)

- The combination of all instruments
shows an overall increase of the production of water, from a few tens
of thousands of kg per day when Rosetta first reached the comet, in
August 2014, to almost 100 000 000 kg per day around perihelion, the
closest point to the Sun along the comet's orbit, in August 2015. In
addition, ROSINA data show that the peak in water production is
followed by a rather steep decrease in the months following perihelion.

- "We were pleasantly surprised to
find such a good agreement between the data collected by all the
various instruments in this unprecedented study of the water production
rate's evolution for a Jupiter-family comet," says Hansen. The
scientists analyzed almost two years' worth of data from ROSINA, which
detects neutral water molecules with its DFMS (Double-Focussing Mass
Spectrometer). "This is by no means trivial: ROSINA performs
measurements locally, at specific points around the comet, and we need
a model to extend them to the entire atmosphere," adds Hansen.

Figure 71: The water production
rate measured at Comet 67P/C-G as a function of the comet's distance
from the Sun (in astronomical units, AU). The measurements are from
ROSINA-DFMS (blue diamonds); MIRO (yellow circles); VIRTIS-H (solid
green triangles); VIRTIS-M (unfilled green triangles); RPC-ICA (red
triangles). The dust production rate, estimated from ground-based
observations, is indicated in tan crosses. The data span the period
between June 2014 and May 2016 (image adapted from Ref. 111)

- The simplest model would be a
spherical distribution of the outgassing centered around the nucleus
but, given the complex shape and season cycle of Comet 67P/C-G, this
would be a very crude approximation. For this reason, the ROSINA team
developed a series of numerical simulations to accurately describe the
comet's production of water, which are presented in a separate study
led by Nicolas Fougere also of the University of Michigan. 112)

- From these
simulations, which showed that the water production rate at a comet
like 67P/C-G is highly inhomogeneous, Hansen and his colleagues derived
an empirical model, which they then used to transform the local ROSINA
measurements into estimates of the overall water production rate.

- The results revealed that, during
the first several months of observations, when the comet was at
distances between 3.5 and 1.7 astronomical units (au) from the Sun,
water was predominantly produced in the comet's northern hemisphere.

Figure 72: The water production
rate predicted by simulations as Comet 67P/C-G approached the Sun, from
August 2014 (left) to May 2015 (right), before the equinox that marked
the end of the northern summer. Images adapted from Hansen et al.
(2016); animations courtesy of K.C. Hansen.

- Then, in May 2015, the equinox
marked the end of the 5.5-year long northern summer and the beginning
of the short and intense southern summer. At that time, the comet was
about 1.7 au from the Sun, and scientists expected that the peak of
water production would drift slowly from the northern to the southern
hemisphere; instead, this transition happened more abruptly than
predicted. This was likely due to the complex shape of the nucleus,
which causes highly variable illumination conditions including
self-shadowing effects.

- As expected, the production of
water peaked between the end of August and early September 2015, about
three weeks after the comet's perihelion, which took place on 13
August, 1.24 au from the Sun. The data hint at possible variations in
the water production rate at this epoch: these might be due to the
spacecraft's motion relative to the comet, but could also be an
indication of actual changes to the outgassing dynamics, and will be
subject of future in-depth investigation.

Figure 73: The water production
rate predicted by simulations after the equinox of Comet 67P/C-G, which
marked the beginning of the southern summer, and covering several weeks
around the comet's perihelion. Images adapted from Hansen et al.
(2016); animations courtesy of K.C. Hansen

- In addition to the ROSINA
measurements, Hansen and his colleagues collated a series of previously
published measurements of the water production rate at 67P/C-G. These
include observations performed with the Microwave Instrument for the
Rosetta Orbiter (MIRO) shortly before and after Rosetta had reached the
comet, data from the Visible and Infrared Thermal Imaging Spectrometer
(VIRTIS) obtained between November 2014 and January 2015, and
measurements from the Ion Composition Analyzer, part of the Rosetta
Plasma Consortium (RPC) suite of instruments, obtained between October
2014 and April 2015.

- RPC-ICA does not detect water directly, but rather measures the ratio of differently ionized Helium ions; since He+ ions arise mainly from collisions between alpha particles (He2+)
from the solar wind and neutral molecules, such as water, found in the
comet's atmosphere, this ratio can be used to estimate the amount of
water produced at the comet.

- Hansen and his collaborators have
found some small discrepancies between the various data sets: for
example, the measurements from ROSINA yield systematically higher
values than those from VIRTIS. One possible reason for this is the
different nature of the two experiments: ROSINA samples the gas in the
coma at the spacecraft's position, while VIRTIS tends to observe closer
to the nucleus, where the water production activity is potentially more
confined than it is further out in the coma. The difference in
measurements techniques and the discrepancy could potentially indicate
an extended source of water in the coma itself, for example icy grains
that are lifted into the coma and turn into gas a few kilometers above
the surface.

- Another difference was found
between the MIRO measurements, which indicate a rising trend in the
water production rate from June to September 2014, and the first months
of ROSINA data, starting in August, pointing to an almost constant rate
in the same period. "This could be explained if a sudden surge in the
water production happened around the time of the first MIRO
measurement, a few weeks before Rosetta's rendezvous with 67P/C-G, and
the beginning of ROSINA observations," says Hansen.

• September 23, 2016: Brief
but powerful outbursts seen from Comet 67P/Churyumov–Gerasimenko
during its most active period last year have been traced back to their
origins on the surface. In the three months centered around the
comet’s closest approach to the Sun, on 13 August 2015,
Rosetta’s cameras captured 34 outbursts. These violent events
were over and above regular jets and flows of material seen streaming
from the comet’s nucleus. The latter switch on and off with
clockwork repeatability from one comet rotation to the next,
synchronized with the rise and fall of the Sun’s illumination. 113)114)

- By contrast,
outbursts are much brighter than the usual jets – sudden, brief,
high-speed releases of dust. They are typically seen only in a single
image, indicating that they have a lifetime shorter than interval
between images – typically 5–30 minutes. A typical outburst
is thought to release 60–260 tons of material in those few
minutes.

- On average, the outbursts around
the closest approach to the Sun occurred once every 30 hours –
about 2.4 comet rotations. Based on the appearance of the dust flow,
they can be divided into three categories. One type is associated with
a long, narrow jet extending far from the nucleus, while the second
involves a broad, wide base that expands more laterally. The third
category is a complex hybrid of the other two.

- “As any given outburst is
short-lived and only captured in one image, we can’t tell whether
it was imaged shortly after the outburst started, or later in the
process,” notes Jean-Baptiste Vincent, lead author of the paper
published in MNRAS (Monthly Notices of the Royal Astronomical Society).
“As a result, we can’t tell if these three types of plume
‘shapes’ correspond to different mechanisms, or just
different stages of a single process. - But if just one process is
involved, then the logical evolutionary sequence is that an initially
long narrow jet with dust is ejected at high speed, most likely from a
confined space. -Then, as the local surface around the exit point is
modified, a larger fraction of fresh material is exposed, broadening
the plume ‘base’. - Finally, when the source region has
been altered so much as not to be able to support the narrow jet
anymore, only a broad plume survives.”

- The OSIRIS cameras on board ESA's
Rosetta spacecraft have monitored the activity of comet
67P-Churyumov-Gerasimenko (67P) across varying heliocentric distances
(4 AU to 1.24 AU) and different seasons on the nucleus (sub solar
latitude between +45º and -55 º). One of the striking
discoveries of Rosetta has been the clockwork repeatability of jets
from one rotation to the next. Jets are very dynamic by nature,
depending on the complex hydrodynamics of the gas and dust streams
interacting with the local topography, and controlled by local thermal
conditions. They grow and fade with the solar illumination as the
nucleus rotates, but the same exact features can be observed from one
rotation to the next. Figure 74 shows an
example of this phenomenon. This, of course, put constraints on the
thermophysics and volatile content of active areas, which need to
ensure the sustainability and repeatability of the jets we observed.

- In the study, monitoring data
acquired by the OSIRIS Narrow Angle and Wide Angle Cameras (NAC &
WAC), as well as Rosetta's navigation camera (NAVCAM) were used to
increase the temporal coverage. Around perihelion, OSIRIS monitoring
campaigns were run on a weekly basis, with a set of images acquired
every 1/2 h for slightly longer than the current nucleus rotation
period (12h18m10s at perihelion). After noticing the first outbursts in
July 2015, the cadence of images was increased in each observation, and
the time was reduced between monitoring campaigns to a few days. In
addition to the OSIRIS data, transient events in the navigation images
were also looked into, acquired about every 4 hours during the whole
mission.

To distinguish between outbursts
and other short lived features, the following definition was
established: An outburst is identified by a sudden brightness increase
in the coma, associated to a release of gas and dust over a duration
very short with respect to the rotation period of the nucleus.
Typically detected in one image only, depending on the observing
cadence. The dust plume is typically one order of magnitude brighter
than the usual jets. Plume morphology as a criterion was not imposed.

Following this definition, 34
events were identified in the data set, listed in a Table. Among them,
26 were detected with OSIRIS NAC, 3 by OSIRIS WAC, and 5 by the NAVCAM.
A visual catalog of the brightest evens is provided in Figure 75.

- Figure 76
shows all outburst sources projected on a topographic map of 67P, and
on a morphological map displaying the regions boundaries. All sources
but one are located in the southern hemisphere, between 0 and -50º
of latitude, i.e. around the sub solar latitude for this period (it
varied from -30 to -55º). This is consistent with previous
observations showing that active sources in general migrate with the
Sun. Outburst sources are not evenly distributed along this latitude.
Some clustering in three main areas were observed: (1) The Anhur-Aker
boundary (big lobe), (2) the Anuket-Sobek boundary (big lobe), and (3)
the Wosret-Maftet boundary (small lobe). These areas are characterized
by steep scarps, cliffs, and pits, which contrast with the overall
flatter morphology of the Southern hemisphere. It is interesting to
note that beyond those three areas, it seems like all outbursts sources
are located close to morphological boundaries, i.e. areas where we
observe discontinuities in the local terrain, either textural or
topographic. This seems to indicate a link between morphology and
outbursts, although it is not clear which one influences the other.

Figure 76:
Maps of all summer outbursts detected by the OSIRIS cameras (blue dots)
and Rosetta's NAVCAM (red dots). The top panel plots the sources over a
topographic maps in which the gray shading represents the local
gravitational slope (white=flat, black=vertical wall). Dotted ellipses
represent the estimated uncertainty for the few outbursts whose source
was not observed directly (image credit: ESA/Rosetta/NAVCAM - CC BYSA
IGO 3.0)

Legend to Figure 76:
Note that this map is a 2D representation of a bi-lobate, strongly
concave object, and therefore presents significant distortions. To
guide the reader, white dashed lines of the boundary of the two lobes
are indicated: the map is centered on the small lobe, the big lobe
covers the left-right-bottom edges of the map, and the contact area
between the two lobes covers mainly the top of the map (regions Hapi,
Neith, Sobek). The 3 main clusters of outbursts sources are located
around longitudes 60º (big lobe), 300º (southern neck), and
315º (small lobe).

• September 9, 2016: Squeezing
out unique scientific observations until the very end, Rosetta’s
thrilling mission will culminate with a descent on 30 September towards
a region of active pits on the comet’s ‘head’. The
region, known as Ma’at, lies on the smaller of the two lobes of
Comet 67P/Churyumov–Gerasimenko. It is home to several active
pits more than 100 m in diameter and 50–60 m in depth –
where a number of the comet’s dust jets originate. 115)116)

- The walls of the pits also exhibit
intriguing meter-sized lumpy structures called
‘goosebumps’, which scientists believe could be the
signatures of early ‘cometesimals’ that assembled to create
the comet in the early phases of Solar System formation.

- Rosetta will
get its closest look yet at these fascinating structures on 30
September: the spacecraft will target a point adjacent to a 130 m-wide,
well-defined pit that the mission team has informally named Deir
el-Medina, after a structure with a similar appearance in an ancient
Egyptian town of the same name.

- Like the archaeological artefacts
found inside the Egyptian pit that tell historians about life in that
town, the comet’s pit contains clues to the geological history of
the region. Rosetta will target a point very close to Deir el-Medina,
within an ellipse about 700 x 500 m.

- Since 9 August, Rosetta has been
flying elliptical orbits that bring it progressively closer to the
comet – on its closest flyby, it may come within 1 km of the
surface, closer than ever before.

- “Although we’ve been
flying Rosetta around the comet for two years now, keeping it operating
safely for the final weeks of the mission in the unpredictable
environment of this comet and so far from the Sun and Earth, will be
our biggest challenge yet,” says Sylvain Lodiot, ESA’s
spacecraft operations manager. “We are already feeling the
difference in gravitational pull of the comet as we fly closer and
closer: it is increasing the spacecraft’s orbital period, which
has to be corrected by small maneuvers. But this is why we have these
flyovers, stepping down in small increments to be robust against these
issues when we make the final approach.”

- The final flyover will be complete
on 24 September. Then a short series of maneuvers needed to line
Rosetta up with the target impact site will be executed over the
following days as it transfers from flying elliptical orbits around the
comet onto a trajectory that will eventually take it to the
comet’s surface on 30 September.

- The collision maneuver will take
place in the evening of 29 September, initiating the descent from an
altitude of about 20 km. Rosetta will essentially free-fall slowly
towards the comet in order to maximize the number of scientific
measurements that can be collected and returned to Earth before its
impact.

- A number of Rosetta’s
scientific instruments will collect data during the descent, providing
unique images and other data on the gas, dust and plasma very close to
the comet. The exact complement of instruments and their operational
timeline remains to be fixed, because it depends on constraints of the
final planned trajectory and the data rate available on the day.

Figure 77: Rosetta’s planned impact site (image credit: ESA)

Figure 78: Planned maneuvers of Rosetta during September for final impact on Comet 67P/Churyumov-Gerasimenko (image credit: ESA)

• September 7, 2016: Rosetta's
dust-analyzing COSIMA (COmetary Secondary Ion Mass Analyzer) instrument
has made the first unambiguous detection of solid organic matter in the
dust particles ejected by Comet 67P/Churyumov-Gerasimenko, in the form
of complex carbon-bearing molecules. 117)118)

- The optical images (Figure 79)
of two of the dust grains collected and analyzed by COSIMA, named
Kenneth and Juliette, which show the signature of carbon-based
organics. They were collected in May and October 2015, respectively.

• September 5, 2016: Less than
a month before the end of the mission, Rosetta’s high-resolution
camera has revealed the Philae lander wedged into a dark crack on Comet
67P/Churyumov–Gerasimenko. 119)120)

Figure 80: The images were taken
on 2 September by the OSIRIS narrow-angle camera as the orbiter came
within 2.7 km of the surface and clearly show the main body of the
lander, along with two of its three legs .The images also provide proof
of Philae’s orientation, making it clear why establishing
communications was so difficult following its landing on 12 November
2014. (image credit: Main image and lander inset: ESA/Rosetta/MPS for
OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context:
ESA/Rosetta/ NavCam – CC BY-SA IGO 3.0)

- “With only a month left of
the Rosetta mission, we are so happy to have finally imaged Philae, and
to see it in such amazing detail,” says Cecilia Tubiana of the
OSIRIS camera team, the first person to see the images when they were
downlinked from Rosetta yesterday.

- “After months of work, with
the focus and the evidence pointing more and more to this lander
candidate, I’m very excited and thrilled that we finally have
this all-important picture of Philae sitting in Abydos,” says
ESA’s Laurence O’Rourke, who has been coordinating the
search efforts over the last months at ESA, with the OSIRIS and Lander
Science Operations and Navigation Center (SONC, CNES) teams.

- Philae was last seen when it first
touched down at Agilkia, bounced and then flew for another two hours
before ending up at a location later named Abydos, on the comet’s
smaller lobe. After three days, Philae's primary battery was exhausted
and the lander went into hibernation, only to wake up again and
communicate briefly with Rosetta in June and July 2015 as the comet
came closer to the Sun and more power was available.

- However,
until today, the precise location was not known. Radio ranging data
tied its location down to an area spanning a few tens of meters, but a
number of potential candidate objects identified in relatively
low-resolution images taken from larger distances could not be analyzed
in detail until recently.

- While most candidates could be
discarded from analysis of the imagery and other techniques, evidence
continued to build towards one particular target, which is now
confirmed in images taken unprecedentedly close to the surface of the
comet.

- At 2.7 km, the resolution of the
OSIRIS narrow-angle camera is about 5 cm/pixel, sufficient to reveal
characteristic features of Philae’s 1 m-sized body and its legs,
as seen in these definitive pictures.

- “This remarkable discovery
comes at the end of a long, painstaking search,” says Patrick
Martin, ESA’s Rosetta Mission Manager. “We were beginning
to think that Philae would remain lost forever. It is incredible we
have captured this at the final hour.”

- "Now that the lander search is
finished we feel ready for Rosetta's landing, and look forward to
capturing even closer images of Rosetta's touchdown site,” adds
Holger Sierks, principal investigator of the OSIRIS camera.

• August 25, 2016: In
unprecedented observations made earlier this year, Rosetta unexpectedly
captured a dramatic comet outburst that may have been triggered by a
landslide. Nine of Rosetta's instruments, including its cameras, dust
collectors, and gas and plasma analyzers, were monitoring the comet
from about 35 km in a coordinated planned sequence when the outburst
happened on 19 February, 2016. 121)122)

- "Over the last year, Rosetta has
shown that although activity can be prolonged, when it comes to
outbursts, the timing is highly unpredictable, so catching an event
like this was pure luck," says Matt Taylor, ESA's Rosetta Project
Scientist. "By happy coincidence, we were pointing the majority of
instruments at the comet at this time, and having these simultaneous
measurements provides us with the most complete set of data on an
outburst ever collected."

- The data were sent to Earth only a
few days after the outburst, but subsequent analysis has allowed a
clear chain of events to be reconstructed, as described in a paper led
by Eberhard Grün of the Max-Planck-Institute for Nuclear Physics,
Heidelberg, accepted for publication in Monthly Notices of the Royal
Astronomical Society.

- A strong brightening of the
comet's dusty coma was seen by the OSIRIS wide-angle camera at 09:40
UTC, developing in a region of the comet that was initially in shadow.

- Over the
next two hours, Rosetta recorded outburst signatures that exceeded
background levels in some instruments by factors of up to a hundred.
For example, between about 10:00–11:00 UTC, ALICE saw the
ultraviolet brightness of the sunlight reflected by the nucleus and the
emitted dust increase by a factor of six, while ROSINA and RPC detected
a significant increase in gas and plasma, respectively, around the
spacecraft, by a factor of 1.5–2.5. In addition, MIRO recorded a
30°C rise in temperature of the surrounding gas.

- Shortly after, Rosetta was blasted
by dust: GIADA recorded a maximum hit count at around 11:15 UTC. Almost
200 particles were detected in the following three hours, compared with
a typical rate of 3–10 collected on other days in the same month.

- At the same time, OSIRIS
narrow-angle camera images began registering dust grains emitted during
the blast. Between 11:10 UTC and 11:40 UTC, a transition occurred from
grains that were distant or slow enough to appear as points in the
images, to those either close or fast enough to be captured as trails
during the exposures.

- In addition, the star trackers,
which are used to navigate and help control Rosetta's attitude,
measured an increase in light scattered from dust particles as a result
of the outburst. - The star trackers are mounted at 90° to the side
of the spacecraft that hosts the majority of science instruments, so
they offered a unique insight into the 3D structure and evolution of
the outburst.

Figure 83: Evolution of a comet
outburst (credit of image: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; all data from Grün et al.
(2016))

- By examining all of the available
data, scientists believe they have identified the source of the
outburst. "From Rosetta's observations, we believe the outburst
originated from a steep slope on the comet's large lobe, in the Atum
region," says Eberhard.

• July 28, 2016: Detailed
analysis of data collected by Rosetta show that comets are the ancient
leftovers of early Solar System formation, and not younger fragments
resulting from subsequent collisions between other, larger bodies.
— Understanding how and when objects like Comet
67P/Churyumov-Gerasimenko took shape is of utmost importance in
determining how exactly they can be used to interpret the formation and
early evolution of our Solar System (Figure 84).123)

- A new study
addressing this question led by Björn Davidsson of the Jet
Propulsion Laboratory, California Institute of Technology in Pasadena
(USA), has been published in Astronomy & Astrophysics. 124)

- If comets are primordial, then
they could help reveal the properties of the solar nebula from which
the Sun, planets and small bodies condensed 4.6 billion years ago, and
the processes that transformed our planetary system into the
architecture we see today.

- The alternative hypothesis is that
they are younger fragments resulting from collisions between older
'parent' bodies such as icy trans-Neptunian objects (TNOs). They would
then provide insight into the interior of such larger bodies, the
collisions that disrupted them, and the process of building new bodies
from the remains of older ones.

- "Either way, comets have been
witness to important Solar System evolution events, and this is why we
have made these detailed measurements with Rosetta – along with
observations of other comets – to find out which scenario is more
likely," says Matt Taylor, ESA's Rosetta project scientist.

- During its two-year sojourn at
Comet 67P/Churyumov-Gerasimenko, Rosetta has revealed a picture of the
comet as a low-density, high-porosity, double-lobed body with extensive
layering, suggesting that the lobes accumulated material over time
before they merged.

- The unusually high porosity of the
interior of the nucleus provides the first indication that this growth
cannot have been via violent collisions, as these would have compacted
the fragile material. Structures and features on different size scales
observed by Rosetta's cameras provide further information on how this
growth may have taken place.

- Earlier work showed that the head
and body were originally separate objects, but the collision that
merged them must have been at low speed in order not to destroy both of
them. The fact that both parts have similar layering also tells us that
they must have undergone similar evolutionary histories and that
survival rates against catastrophic collision must have been high for a
significant period of time.

- Merging events may also have
happened on smaller scales. For example, three spherical 'caps' have
been identified in the Bastet region on the small comet lobe, and
suggestions are that they are remnants of smaller cometesimals that are
still partially preserved today.

- At even smaller scales of just a
few meters across, there are the so-called 'goosebumps' and 'clod'
features, rough textures observed in numerous pits and exposed cliff
walls in various locations on the comet.

- While it is possible that this
morphology might arise from fracturing alone, it is actually thought to
represent an intrinsic 'lumpiness' of the comet's constituents. That
is, these 'goosebumps' could be showing the typical size of the
smallest cometesimals that accumulated and merged to build up the
comet, made visible again today through erosion due to sunlight.

- According to theory, the speeds at
which cometesimals collide and merge change during the growth process,
with a peak when the lumps have sizes of a few meters. For this reason,
meter-sized structures are expected to be the most compact and
resilient, and it is particularly interesting that the comet material
appears lumpy on that particular size scale.

- Further lines of evidence include
spectral analysis of the comet's composition showing that the surface
has experienced little or no in situ alteration by liquid water, and
analysis of the gases ejected from sublimating ices buried deeper
within the surface, which finds the comet to be rich in supervolatiles
such as carbon monoxide, oxygen, nitrogen and argon.

- These observations imply that
comets formed in extremely cold conditions and did not experience
significant thermal processing during most of their lifetimes. Instead,
to explain the low temperatures, survival of certain ices and retention
of supervolatiles, they must have accumulated slowly over a significant
time period.

- "While larger TNOs in the outer
reaches of the Solar System appear to have been heated by short-lived
radioactive substances, comets don't seem to show similar signs of
thermal processing. We had to resolve this paradox by taking a detailed
look at the time line of our current Solar System models, and consider
new ideas," says Björn. Björn and colleagues propose that the
larger members of the TNO population formed rapidly within the first
one million years of the solar nebula, aided by turbulent gas streams
that rapidly accelerated their growth to sizes of up to 400 km.

- Around three million years into
the Solar System's history, gas had disappeared from the solar nebula,
only leaving solid material behind. Then, over a much longer period of
around 400 million years, the already massive TNOs slowly accreted
further material and underwent compaction into layers, their ices
melting and refreezing, for example. Some TNOs even grew into Pluto or
Triton-sized objects.

- Comets took a different path.
After the rapid initial growth phase of the TNOs, leftover grains and
'pebbles' of icy material in the cold, outer parts of the solar nebula
started to come together at low velocity, yielding comets roughly 5 km
in size by the time gas has disappeared from the solar nebula. The low
speeds at which the material accumulated led to objects with fragile
nuclei with high porosity and low density. - This slow growth also
allowed comets to preserve some of the oldest, volatile-rich material
from the solar nebula, since they were able to release the energy
generated by radioactive decay inside them without heating up too much.

- The larger
TNOs played a further role in the evolution of comets. By 'stirring'
the cometary orbits, additional material was accreted at somewhat
higher speed over the next 25 million years, forming the outer layers
of comets. The stirring also made it possible for the few
kilometer-sized objects in size to bump gently into each other, leading
to the bi-lobed nature of some observed comets.

- "Comets do not appear to display
the characteristics expected for collisional rubble piles, which result
from the smash-up of large objects like TNOs. Rather, we think they
grew gently in the shadow of the TNOs, surviving essentially undamaged
for 4.6 billion years," concludes Björn. "Our new model explains
what we see in Rosetta's detailed observations of its comet, and what
had been hinted at by previous comet flyby missions."

- "Comets really are the
treasure-troves of the Solar System," adds Matt. "They give us
unparalleled insight into the processes that were important in the
planetary construction yard at these early times and how they relate to
the Solar System architecture that we see today."

• On July 27, 2016, the ESS
(Electrical Support System) on Rosetta, which is used to communicate
with Philae, will be switched off to save energy before September 30,
the day the Rosetta mission will come to an end. 125)

• July 15, 2016: This CometWatch image (Figure 85)
was taken with Rosetta's NAVCAM on 9 July 2016, when the spacecraft was
11.7 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The
close-up view shows a portion of the Khonsu region on the larger of the
two comet lobes. Khonsu is part of the southern hemisphere of 67P/C-G. 126)

- The image
reveals a variety of fractured and smooth terrains, with a great number
of boulders of all sizes, including several large ones. It also
includes a three-layered structure with a balancing boulder on top,
which was also portrayed in previous images, for example the NAVCAM
view featured as CometWatch 13 June, which shows the same region but
from a broader perspective.

• June 30, 2016: Rosetta is
set to complete its mission in a controlled descent to the surface of
its comet on 30 September. The mission is coming to an end as a result
of the spacecraft's ever-increasing distance from the Sun and Earth. It
is heading out towards the orbit of Jupiter, resulting in significantly
reduced solar power to operate the craft and its instruments, and a
reduction in bandwidth available to downlink scientific data. 127)

- Combined with an ageing spacecraft
and payload that have endured the harsh environment of space for over
12 years – not least two years close to a dusty comet –
this means that Rosetta is reaching the end of its natural life.

- Unlike in 2011, when Rosetta was
put into a 31-month hibernation for the most distant part of its
journey, this time it is riding alongside the comet. Comet
67P/Churyumov-Gerasimenko's maximum distance from the Sun (over 850
million km) is more than Rosetta has ever journeyed before. The result
is that there is not enough power at its most distant point to
guarantee that Rosetta's heaters would be able to keep it warm enough
to survive.

- Instead of risking a much longer
hibernation that is unlikely to be survivable, and after consultation
with Rosetta's science team in 2014, it was decided that Rosetta would
follow its lander Philae down onto the comet.

- The final hours of descent will
enable Rosetta to make many once-in-a-lifetime measurements, including
very-high-resolution imaging, boosting Rosetta's science return with
precious close-up data achievable only through such a unique
conclusion. — Communications will cease, however, once the
orbiter reaches the surface, and its operations will then end.

- "We're trying to squeeze as many
observations in as possible before we run out of solar power," says
Matt Taylor, ESA Rosetta project scientist. "30 September will mark the
end of spacecraft operations, but the beginning of the phase where the
full focus of the teams will be on science. That is what the Rosetta
mission was launched for and we have years of work ahead of us,
thoroughly analyzing its data."

- Rosetta's operators will begin
changing the trajectory in August ahead of the grand finale such that a
series of elliptical orbits will take it progressively nearer to the
comet at its closest point.

- "Planning this phase is in fact
far more complex than it was for Philae's landing," says Sylvain
Lodiot, ESA Rosetta spacecraft operations manager. "The last six weeks
will be particularly challenging as we fly eccentric orbits around the
comet – in many ways this will be even riskier than the final
descent itself. - "The closer we get to the comet, the more influence
its non-uniform gravity will have, requiring us to have more control on
the trajectory, and therefore more maneuvers – our planning
cycles will have to be executed on much shorter timescales."

- A number of dedicated maneuvers in
the closing days of the mission will conclude with one final trajectory
change at a distance of around 20 km about 12 hours before impact, to
put the spacecraft on its final descent.

- In the meantime, science will
continue as normal, although there are still many risks ahead. Last
month, the spacecraft experienced a 'safe mode' while only 5 km from
the comet as a result of dust confusing the navigation system. Rosetta
recovered, but the mission team cannot rule out this happening again
before the planned end of the mission.

• May 27, 2016: Ingredients
regarded as crucial for the origin of life on Earth have been
discovered at the comet that ESA's Rosetta spacecraft has been probing
for almost two years. They include the amino acid glycine, which is
commonly found in proteins, and phosphorus, a key component of DNA and
cell membranes.128)129)

- Scientists have long debated the
important possibility that water and organic molecules were brought by
asteroids and comets to the young Earth after it cooled following its
formation, providing some of the key building blocks for the emergence
of life. While some comets and asteroids are already known to have
water with a composition like that of Earth's oceans, Rosetta found a
significant difference at its comet – fuelling the debate on
their role in the origin of Earth's water.

- But new results reveal that comets
nevertheless had the potential to deliver ingredients critical to
establish life as we know it. Amino acids are biologically important
organic compounds containing carbon, oxygen, hydrogen and nitrogen, and
form the basis of proteins.

- Hints of the simplest amino acid,
glycine, were found in samples returned to Earth in 2006 from Comet
Wild-2 by NASA's Stardust mission. However, possible terrestrial
contamination of the dust samples made the analysis extremely
difficult.

- Now, Rosetta has made direct, repeated detections of glycine in the fuzzy atmosphere or 'coma' of its comet. "This is the first unambiguous detection of glycine at a comet,"
says Kathrin Altwegg, principal investigator of the ROSINA instrument
that made the measurements, and lead author of the paper. "At the
same time, we also detected certain other organic molecules that can be
precursors to glycine, hinting at the possible ways in which it may
have formed."

- The
measurements were made before the comet reached its closest point to
the Sun – perihelion – in August 2015 in its 6.5 year
orbit. The first detection was made in October 2014 while Rosetta was
just 10 km from the comet. The next occasion was during a flyby in
March 2015, when it was 30–15 km from the nucleus.

- A sample mass spectrum at 75, 45, 31, and 30 dalton is shown in Figure86.
The number of ionized particles registered on the detector is given as
a function of the position on the detector, which corresponds to
mass/charge ratio (m/z). Glycine (C2H5NO2), methylamine (CH5N), and ethylamine (C2H7N) can be found on mass 75, 31, and 45 dalton, respectively.

- Glycine was also seen on other
occasions associated with outbursts from the comet in the month leading
up to perihelion, when Rosetta was more than 200 km from the nucleus
but surrounded by a lot of dust. "We see a strong link between
glycine and dust, suggesting that it is probably released perhaps with
other volatiles from the icy mantles of the dust grains once they have
warmed up in the coma," says Kathrin.

- Glycine turns into gas only when
it reaches temperatures just below 150°C, meaning that usually
little is released from the comet's surface or subsurface because of
the low temperatures. This accounts for the fact that Rosetta does not
always detect it. "Glycine is the only amino acid that is known to
be able to form without liquid water, and the fact we see it with the
precursor molecules and dust suggests it is formed within interstellar
icy dust grains or by the ultraviolet irradiation of ice, before
becoming bound up and conserved in the comet for billions of years," adds Kathrin.

- Another exciting detection made by
Rosetta and described in the paper is of phosphorus, a key element in
all known living organisms. For example, it is found in the structural
framework of DNA, in cell membranes and in transporting chemical energy
within cells for metabolism. "There is still a lot of uncertainty
regarding the chemistry on early Earth and there is of course a huge
evolutionary gap to fill between the delivery of these ingredients via
cometary impacts and life taking hold," says co-author Hervé Cottin. "But
the important point is that comets have not really changed in 4.5
billion years: they grant us direct access to some of the ingredients
that likely ended up in the prebiotic soup that eventually resulted in
the origin of life on Earth."

- "The multitude of organic
molecules already identified by Rosetta, now joined by the exciting
confirmation of fundamental ingredients like glycine and phosphorous,
confirms our idea that comets have the potential to deliver key
molecules for prebiotic chemistry," says Matt Taylor, ESA's Rosetta project scientist.

• April 18, 2016: Captured in this curious view (Figure 87)
are the Anuket region and its surroundings on Comet
67P/Churyumov–Gerasimenko. The image was taken by Rosetta’s
navigation camera (NavCam) on 13 March 2016, from a distance of 17 km,
and measures about 1.5 km across. 130)

- However, if
we briefly suspend disbelief and set our imagination free, we might be
tricked into recognizing the profile of a face, with the forehead and
eyebrows on the left, a nose pointing upwards and even the hint of a
smile. This is an effect of pareidolia, a psychological phenomenon
whereby humans tend to identify familiar shapes in the vague patterns
of random images.

- In reality, the fictional face
shows the rough landscape of Anuket, a region of rugged terrains on the
small comet lobe and declining towards the large lobe, which is located
beyond the lower-right corner of the image. What might appear as a
forehead, towards the left, is in fact the surface of Serqet, a small
region comprising flat and smooth terrains and a few boulders. The
sharp cliff separating Serqet and Anuket contributes to the optical
illusion, suggesting the profile of an eye socket and eyebrow.

- Along the sharp boundary between
Serqet and Anuket – the eyebrow – is a round, crescent
feature, known as C. Alexander Gate. It is dedicated to Dr Claudia J.
Alexander, who was the US Rosetta Project Scientist and passed away in
July 2015.

- The image also depicts Serqet
casting a dramatic shadow onto parts of Ma’at, the smooth, dusty
region visible in the lower-right part of the image. Another striking
element is the border between Ma’at and Anuket, highlighting the
difference between the almost featureless appearance of Ma’at and
the rugged terrains of Anuket.

- A hint of the Hathor region is
also visible, albeit cast in shadow, on the lower part of the image.
You can use the comet viewer tool to aid navigation around the
comet’s regions.

- Rosetta is currently around 30 km
from the nucleus, and will continue studying the comet from up close
until the end of September, when it will be maneuvered into a
controlled impact on the comet.

Figure 87:
Image of the Anuket region and its surroundings on Comet
67P/Churyumov–Gerasimenko, acquired on March 13, 2016 with the
NAVCAM from a distance of 17 km (image credit: ESA/Rosetta/NAVCAM
– CC BY-SA IGO 3.0)

• April
7, 2016: Comet 67P/Churyumov-Gerasimenko was seen changing color and
brightness by Rosetta’s VIRTIS (Visible and InfraRed Thermal
Imaging Spectrometer), as more water-ice was exposed near its surface
as it moved close to the Sun between August and November 2014. 131)132)

- In the three-month study period
the comet moved from about 542 million km to 438 million km from the
Sun, and the spacecraft-to-comet distance varied from about 100 km to
10 km, resulting in a range of illumination conditions and viewing
geometries. In general, the darkest portions of the comet, containing
dry dust made out of a mixture of minerals and organics, reflect light
at redder wavelengths, while active regions and the occasional ice-rich
exposure is bluer.

- The VIRTIS study shows that even
in the first three months of study at the comet, global average changes
are noticeable, with an overall trend of the comet becoming brighter
and more water-ice-rich. This is particularly notable in the Imhotep
region, which becomes overall bluer over time.

Figure 88: Illustration of the comet's orbit in the period August-November 2014 (image credit: ESA/ATG medialab)

Figure 89:
Comet 67p/Churyumov-Gerasimenko was seen changing color and brightness
as more water-ice was exposed near its surface as it moved closer to
the Sun between August and November 2014 (image credit:
ESA/Rosetta/VIRTIS/INAF-IAPS/OBS De PARIS-LESIA/DLR; G. Filacchione et
al., 2016)

Legend to Figure 89:
Variation of visible spectral slope (color) over time. Red corresponds
to more organic-rich material; blue indicates more active regions (such
as Hapi) and water-ice rich exposures. The transition from redder to
bluer spectra is most clearly seen in the Imhotep region.

• March
11, 2016: ESA’s Rosetta spacecraft has revealed a surprisingly
large region around its host comet devoid of any magnetic field. The
Rosetta magnetometer RPC-MAG has been exploring the plasma environment
of comet 67P/Churyumov-Gerasimenko since August 2014. The first months
were dominated by low-frequency waves which evolved into more complex
features. However, at the end of July 2015, close to perihelion, the
magnetometer detected a region that did not contain any magnetic field
at all. 133)134)

- These signatures match the
appearance of a diamagnetic cavity as was observed at comet 1P/Halley
in 1986 when ESA's Giotto found a vast magnetic-free region extending
more than 4000 km from the nucleus. This was the first observation of
something that scientists had until then only thought about but had
never seen.

- The cavity at Rosetta is more
extended than previously predicted by models and features unusual
magnetic field configurations, which need to be explained. According to
the team's analysis of the data acquired by the Rosetta Plasma
Consortium instrumentation confirms the existence of a diamagnetic
cavity. However, the size is larger than predicted by simulations. One
possible explanation are instabilities that are propagating along the
cavity boundary and possibly a low magnetic pressure in the solar wind.
This conclusion is supported by a change in sign of the Sun-pointing
component of the magnetic field. Evidence also indicates that the
cavity boundary is moving with variable velocities ranging from 230-500
m/s.

Legend to Figure 90:
The magnetic field-free region, shown here in blue, is caused by the
interaction of the comet and the solar wind, shown in purple. The solar
wind is a flow of electrically charged particles streaming from the Sun
(located beyond the left edge of the image) and carrying its magnetic
field across the Solar System. When it approaches the comet, which is
pouring lots of gas into space, its flow is obstructed and slowed.
Eventually, the solar wind stops entirely, diverting its flow around
the comet, and also its magnetic field is unable to penetrate the
environment around the comet, creating a region devoid of magnetic
field called a diamagnetic cavity. — Scientists expected that
such a diamagnetic cavity could form at Rosetta’s comet around
perihelion, but only extend to 50–100 km from the nucleus, and
since the spacecraft was at greater distances from the comet at the
time, they did not expect to detect it. Instead, they measured almost
700 magnetic field-free regions with the RPC-MAG magnetometer on
Rosetta since June 2015, revealing that the cavity is much bigger than
expected. The reason for that is likely an oscillating perturbation, or
instability (thin wiggly line), that propagates and gets amplified
along the boundary (thick line) between the solar wind and the magnetic
field-free cavity, causing the latter to grow in size and allowing
Rosetta to detect it.

- Prior to Rosetta arriving at Comet
67P/Churyumov-Gerasimenko, scientists had hoped to observe such a
magnetic field-free region in the environment of this comet. The
spacecraft carries a magnetometer as part of the Rosetta Plasma
Consortium suite of sensors (RPC-MAG), whose measurements were already
used to demonstrate that the comet nucleus is not magnetized.

- However, since Rosetta’s
comet is much less active than Comet Halley, the scientists predicted
that a diamagnetic cavity could form only in the months around
perihelion – the closest point to the Sun on the comet’s
orbit – but that it would extend only 50–100 km from the
nucleus. During 2015, the increased amounts of dust dragged into space
by the outflowing gas became a significant problem for navigation close
to the comet. To keep Rosetta safe, trajectories were chosen such that
by the end of July 2015, a few weeks before perihelion, it was some 170
km away from the nucleus. As a result, scientists considered that
detecting signs of the magnetic field-free bubble would be impossible.

- “We
had almost given up on Rosetta finding the diamagnetic cavity, so we
were astonished when we eventually found it,” says Charlotte
Götz of the Institute for Geophysics and extraterrestrial Physics
in Braunschweig, Germany. Charlotte is the lead author of a new study,
published in the journal Astronomy and Astrophysics. “We were
able to detect the cavity, on many occasions, because it is much bigger
and dynamic than we had expected,” adds Charlotte.

Figure 91: The decrease in
magnetic field strength measured by Rosetta’s RPC-MAG instrument
at Comet 67P/Churyumov–Gerasimenko on 26 July 2015 at a distance
of about 170 km from the comet (image credit: ESA/Rosetta/RPC/IGEP/IC)

• Feb. 12, 2016: Silent since
its last call to mothership Rosetta seven months ago, the Philae lander
is facing conditions on Comet 67P/Churyumov–Gerasimenko from
which it is unlikely to recover. 135)136)

- Rosetta, which continues its
scientific investigations at the comet until September before its own
comet-landing finale, has in recent months been balancing science
observations with flying dedicated trajectories optimized to listen out
for Philae. But the lander has remained silent since 9 July 2015.

- “The chances for Philae to
contact our team at our lander control center are unfortunately getting
close to zero,” says Stephan Ulamec, Philae project manager at
the German Aerospace Center, DLR. “We are not sending commands
any more and it would be very surprising if we were to receive a signal
again.”

- Philae’s team of expert
engineers and scientists at the German, French and Italian space
centers and across Europe have carried out extensive investigations to
try to understand the status of the lander, piecing together clues
since it completed its first set of scientific activities after its
historic landing on 12 November 2014.

- A story with
incredible twists and turns unfolded on that day. In addition to a
faulty thruster, Philae also failed to fire its harpoons and lock
itself onto the surface of the comet after its seven-hour descent,
bouncing from its initial touchdown point at Agilkia, to a new landing
site, Abydos, over 1 km away. The precise location of the lander has
yet to be confirmed in high-resolution images.

- A reconstruction of the flight of
the lander suggested that it made contact with the comet four times
during its two-hour additional flight across the small comet lobe.
After bouncing from Agilkia it grazed the rim of the Hatmehit
depression, bounced again, and then finally settled on the surface at
Abydos (Figure 115).

- Even after this unplanned
excursion, the lander was still able to make an impressive array of
science measurements, with some even as it was flying above the surface
after the first bounce.

- Once the lander had made its final
touchdown, science and operations teams worked around the clock to
adapt the experiments to make the most of the unanticipated situation.
About 80% of its initial planned scientific activities were completed.

- In the 64 hours following its
separation from Rosetta, Philae took detailed images of the comet from
above and on the surface, sniffed out organic compounds, and profiled
the local environment and surface properties of the comet, providing
revolutionary insights into this fascinating world.

- But with insufficient sunlight
falling on Philae’s new home to charge its secondary batteries,
the race was on to collect and transmit the data to Rosetta and across
510 million km of space back to Earth before the lander’s primary
battery was exhausted as expected. Thus, on the evening of 14–15
November 2014, Philae fell into hibernation.

- The last images of Philae will
probably be acquired in the summer of 2016, when the Rosetta spacecraft
images the lander during close fly-bys. "When we see how Philae is
positioned, we will be able to better interpret certain data, such as
the measurements of the CONSERT radar experiment." In approximately six
years, Philae and Rosetta, which will be landed on the comet in
September 2016 at the end of its mission, will be closer to Earth
– and Comet 67/P Churyumov-Gerasimenko will have circled the Sun
once again.

• Feb. 4, 2016: There are no
large caverns inside Comet 67P/Churyumov-Gerasimenko. ESA’s
Rosetta mission has made measurements that clearly demonstrate this,
solving a long-standing mystery. Comets are the icy remnants left over
from the formation of the planets 4.6 billion years ago. A total of
eight comets have now been visited by spacecraft and, thanks to these
missions, we have built up a picture of the basic properties of these
cosmic time capsules. While some questions have been answered, others
have been raised. 137)

- Comets are known to be a mixture
of dust and ice, and if fully compact, they would be heavier than
water. However, previous measurements have shown that some of them have
extremely low densities, much lower than that of water ice. The low
density implies that comets must be highly porous. But is the porosity
because of huge empty caves in the comet’s interior or it is a
more homogeneous low-density structure?

- In a new study led by Martin
Pätzold, the authors have shown that Comet
67P/Churyumov-Gerasimenko is also a low-density object, but they have
also been able to rule out a cavernous interior. This result is
consistent with earlier results from Rosetta’s CONSERT radar
experiment showing that the double-lobed comet’s
‘head’ is fairly homogenous on spatial scales of a few tens
of meters. 138)

- The most reasonable explanation
then is that the comet’s porosity must be an intrinsic property
of dust particles mixed with the ice that make up the interior. In
fact, earlier spacecraft measurements had shown that comet dust is
typically not a compacted solid, but rather a ‘fluffy’
aggregate, giving the dust particles high porosity and low density, and
Rosetta’s COSIMA and GIADA instruments have shown that the same
kinds of dust grains are also found at 67P/Churyumov-Gerasimenko.

- Pätzold’s study team
made their discovery by using the RSI (Radio Science Experiment) to
study the way the Rosetta orbiter is pulled by the gravity of the
comet, which is generated by its mass. The effect of the gravity on the
movement of Rosetta is measured by changes in the frequency of the
spacecraft’s signals when they are received at Earth. It is a
manifestation of the Doppler effect, produced whenever there is
movement between a source and an observer, and is the same effect that
causes emergency vehicle sirens to change pitch as they pass by.

- In this case, Rosetta was being
pulled by the gravity of the comet, which changed the frequency of the
radio link to Earth. ESA’s 35 m antenna at the New Norcia ground
station in Australia is used to communicate with Rosetta during routine
operations. The variations in the signals it received were analyzed to
give a picture of the gravity field across the comet. Large internal
caverns would have been noticeable by a tell-tale drop in acceleration.

- ESA’s Rosetta mission is the
first to perform this difficult measurement for a comet.
“Newton’s law of gravity tells us that the Rosetta
spacecraft is basically pulled by everything,” says Martin
Pätzold, the principal investigator of the RSI experiment.
“In practical terms, this means that we had to remove the
influence of the Sun, all the planets – from giant Jupiter to the
dwarf planets – as well as large asteroids in the inner asteroid
belt, on Rosetta’s motion, to leave just the influence of the
comet. Thankfully, these effects are well understood and this is a
standard procedure nowadays for spacecraft operations.”

- Next, the
pressure of the solar radiation and the comet’s escaping gas tail
has to be subtracted. Both of these ‘blow’ the spacecraft
off course. In this case, Rosetta’s ROSINA instrument is
extremely helpful as it measures the gas that is streaming past the
spacecraft. This allowed Pätzold and his colleagues to calculate
and remove those effects too.

- Whatever motion is left is due to
the comet’s mass. For Comet 67P/Churyumov-Gerasimenko, this gives
a mass slightly less than 10 billion tons. Images from the OSIRIS
camera have been used to develop models of the comet’s shape and
these give the volume as around 18.7 km3, meaning that the density is 533 kg/m3.

- Extracting the details of the
interior was only possible through a piece of cosmic good luck. The
comet’s strange shape was revealed as Rosetta drew nearer for its
rendezvous in August 2014. Luckily for RSI, the double-lobed structure
meant that the differences in the gravity field would be much more
pronounced, and therefore easier to measure from far away.

- In September 2016, Rosetta will be
guided to a controlled impact on the surface of the comet. The maneuver
will provide a unique challenge for the flight dynamics specialists at
ESA/ESOC (European Space Operations Center) in Darmstadt, Germany. As
Rosetta gets nearer and nearer, the complex gravity field of the comet
will make navigating harder and harder. But for RSI, its measurements
will increase in precision. This could allow the team to check for
caverns just a few hundred meters across.

• January 13, 2016: Observations made shortly after Rosetta’s arrival at its target comet in 2014 have provided definitive confirmation of the presence of water ice.
Although water vapor is the main gas seen flowing from comet
67P/Churyumov–Gerasimenko, the great majority of ice is believed
to come from under the comet’s crust, and very few examples of
exposed water ice have been found on the surface. However, a detailed
analysis by Rosetta’s VIRTIS infrared instrument reveals the
composition of the comet’s topmost layer: it is primarily coated
in a dark, dry and organic-rich material but with a small amount of
water ice mixed in. 139)140)

- In the latest study, which focuses
on scans between September and November 2014, the team confirms that
two areas several tens of meters across in the Imhotep region that
appear as bright patches in visible light, do indeed include a
significant amount of water ice. The ice is associated with cliff walls
and debris falls, and was at an average temperature of about
–120ºC at the time.

- In those regions, pure water ice
was found to occupy around 5% of each pixel sampling area, with the
rest made up of the dark, dry material. The abundance of ice was
calculated by comparing Rosetta’s VIRTIS infrared measurements to
models that consider how ice grains of different sizes might be mixed
together in one pixel.

- The data reveal two different
populations of grains: one is several tens of micrometers in diameter,
while the other is larger, around 2 mm. These sizes contrast with the
very small grains, just a few micrometers in diameter, found in the
Hapi region on the ‘neck’ of the comet, as observed by
VIRTIS in a different study.

- The Rosetta scientists are now
analyzing data captured later in the mission, as the comet moved closer
to the Sun in mid-2015, to see how the amount of ice exposed on the
surface evolved as the heating increased.

Legend to Figure 92:
The main image was taken on 17 September 2014 from a distance of about
28.8 km from the comet center. The two insets show oblique views of the
two icy exposures. The left hand image was taken on 20 September 2014
from a distance of 27.9 km. The right hand image was taken on 15
September 2014 from a distance of 29.9 km.

• Dec. 31, 2015: Images from Rosetta taken over the holiday period. 141)

Figure 93:
The single-frame OSIRIS narrow-angle camera image was taken on 31
December 2015, when Rosetta was 79.6 km from the nucleus of Comet
67P/Churyumov–Gerasimenko. The scale is 1.44 m/pixel (image
credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

• Nov. 30, 2015: A new 3D
shape model of Comet 67P/Churyumov-Gerasimenko has been released by
ESA’s Rosetta archive team (Figure 94).
The model includes images taken by Rosetta’s NAVCAM up until
mid-late July 2015, and reveals parts of the comet’s southern
hemisphere that were not included in earlier shape models. 142)

• November 12, 2015: As
announced in June 2015 along with confirmation of the mission’s
extension, Rosetta teams are planning to end the operational phase of
the mission in a controlled impact of the orbiter on the surface of
Comet 67P/Churyumov-Gerasimenko at the end of September 2016. While
the specific details of the trajectories and impact site are still
under discussion, ESA’s Rosetta Spacecraft Operations Manager
Sylvain Lodiot, Project Scientist Matt Taylor, and mission manager
Patrick Martin, share some background information on the planning of
this dramatic mission finale. 143)

• Nov.
12, 2015: One year since Philae made its historic landing on a comet,
mission teams remain hopeful for renewed contact with the lander, while
also looking ahead to next year’s grand finale: making a
controlled impact of the Rosetta orbiter on the comet. - Rosetta
arrived at Comet 67P/Churyumov–Gerasimenko on 6 August 2014, and
after an initial survey and selection of a landing site, Philae was
delivered to the surface on 12 November. 144)145)146)

- After touching down in the Agilkia region as planned (Figure 95),
Philae did not secure itself to the comet, and it bounced to a new
location in Abydos. Its flight across the surface is depicted in a new
animation, using data collected by Rosetta and Philae to reconstruct
the lander’s rotation and attitude.

- In the year since landing, a
thorough analysis has also now been performed on why Philae bounced.
There were three methods to secure it after landing: ice screws,
harpoons and a small thruster. The ice screws were designed with
relatively soft material in mind, but Agilkia turned out to be very
hard and they did not penetrate the surface.

- The harpoons were capable of
working in both softer and harder material. They were supposed to fire
on contact and lock Philae to the surface, while a thruster on top of
the lander was meant to push it down to counteract the recoil from the
harpoon. - Attempts to arm the thruster the night before failed: it is
thought that a seal did not open, although a sensor failure cannot be
excluded.

- Then, on landing, the harpoons
themselves did not fire. “It seems that the problem was either
with the four ‘bridge wires’ taking current to ignite the
explosive that triggers the harpoons, or the explosive itself, which
may have degraded over time,” explains Stephan Ulamec, Philae
lander manager at the DLR German Aerospace Center. “In any case,
if we can regain contact with Philae, we might consider an attempt to
retry the firing.”

- The reason is scientific: the
harpoons contain sensors that could measure the temperature below the
surface. Despite the unplanned bouncing, Philae completed 80% of its
planned first science sequence before falling into hibernation in the
early hours of 15 November when the primary battery was exhausted.
There was not enough sunlight in Philae’s final location at
Abydos to charge the secondary batteries and continue science
measurements.

- The hope was that as the comet
moved nearer to the Sun, heading towards closest approach in August,
there would be enough energy to reactivate Philae. Indeed, contact was
made with the lander on 13 June 2015 but only eight intermittent
contacts were made up to 9 July. The problem was that the increasing
sunlight also led to increased activity on the comet, forcing Rosetta
to retreat to several hundred kilometers for safety, well out of range
with Philae.

- However, over the past few weeks,
with the comet’s activity now subsiding, Rosetta has started to
approach again. This week it reached 200 km, the limit for making good
contact with Philae, and today it dips to within 170 km.

- In the meantime, the lander teams
have continued their analysis of the data returned during the contacts
in June and July, hoping to understand the status of Philae when it
first woke up from hibernation. “We had already determined that
one of Philae’s two receivers and one of the two transmitters
were likely no longer working,” says Koen Geurts, Philae’s
technical manager at DLR’s Lander Control Center in Cologne,
Germany, “and it now seems that the other transmitter is
suffering problems. Sometimes it did not switch on as expected, or it
switched off too early, meaning that we likely missed possible
contacts.” The team is taking this new information into account
to determine the most promising strategy to regain regular contact. But
it’s a race against time: with the comet now heading out beyond
the orbit of Mars, temperatures are falling.

Figure 95:
The area surrounding Philae’s first touchdown point, Agilkia
(circled) on comet 67P/Churyumov–Gerasimenko. The large
depression is the Hatmehit region. The dashed line marks the
comet’s equator. This image is a composite of five frames from
the OSIRIS narrow-angle camera (image credit: ESA/Rosetta/MPS for
OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

• October 2015 (Flight
dynamics analysis for the approach of Rosetta with the comet): When the
Rosetta spacecraft approached comet Churyumov-Gerasimenko in August
2014 after ten years of flight, almost nothing was known about its
target’s gravity field and rotation, except for the rotation
period. Since knowledge of these parameters is essential for trajectory
design, a flexible strategy had to be devised to cope with a large
range of possible values. In addition, the strategy had to be robust
against contingencies like missed orbit maneuvers. The first
simulations showed that the initial plan was not feasible. After
several iterations, a strategy meeting these requirements was found. It
consisted of a Comet Approach Phase, where the relative velocity was
gradually reduced by orbit maneuvers of decreasing size, and an Initial
Characterization Phase, where the physical parameters of the comet were
determined from a sequence of hyperbolic arcs forming a pyramid-like
orbit. These parameters would subsequently be used to adapt the
trajectories for the next phase of the mission, where the spacecraft
would be put into a bound orbit around the comet. The strategy chosen
was successfully validated by a navigation analysis in which the
operations that would be performed during the real approach were
simulated, and finally confirmed when it was put into action during the
real operations. 147)

- In
preparation for the approach of spacecraft Rosetta to comet
67P/Churyumov-Gerasimenko, an extensive number of simulations were
performed. These included both closed-loop and open-loop simulations,
where the more realistic closed-loop simulations were used to validate
the idea of open-loop simulations, which are less realistic but require
less effort.

- The first simulations clearly
indicated that the original plan, which foresaw a direct insertion into
a bound orbit at the end of the approach phase, was not feasible since
the comet mass and attitude could not be determined with sufficient
accuracy before the insertion maneuver. Instead, after several
iterations, a new strategy was devised: The initial characterization of
the comet was done from hyperbolic arcs at cometocentric distances
between 50 and 120 km. The velocity in these arcs was chosen such that
the influence of the initially poorly known comet gravity force on the
spacecraft orbit was large enough to provide an accurate determination
of the comet mass, but still small enough to allow a reasonably
accurate orbit prediction. Orbit maneuvers were performed regularly to
keep the spacecraft close to the comet, in such a way that the
comet-spacecraft vector formed two pyramids with the nucleus at the
apex.

- From these pyramid orbits, the
mass, rotational state and shape of the comet could be estimated.
Moreover, landmarks on the surface of the nucleus could be identified
with sufficient accuracy for a safe insertion into a bound orbit.
Following the insertion, the spacecraft entered the Global Mapping and
the Close Observation phases, in which the knowledge about the comet
properties was further improved by observations from successively
closer orbits around the comet.

- The validity of the revised
strategy was finally fully confirmed when it was put into action during
the actual operations, without the need for any modifications. All
orbit maneuvers during the approach performed well within the expected
uncertainties (148)149)),
the comet was detected at the earliest possible stage using the OSIRIS
NAC and the derivation of optical measurements from the camera images
worked flawlessly. 150)151)

• October 28, 2015:
ESA’s Rosetta spacecraft has made the first in situ detection of
oxygen molecules outgassing from a comet, a surprising observation that
suggests they were incorporated into the comet during its formation.
Rosetta has been studying Comet 67P/Churyumov–Gerasimenko for
over a year and has detected an abundance of different gases pouring
from its nucleus. Water vapor, carbon monoxide and carbon dioxide are
the most prolific, with a rich array of other nitrogen-, sulphur- and
carbon-bearing species, and even ‘noble gases’ also
recorded. 152)153)

- Oxygen is the third most abundant element in the Universe, but the simplest molecular version of the gas, O2,
has proven surprisingly hard to track down, even in star-forming
clouds, because it is highly reactive and readily breaks apart to bind
with other atoms and molecules. For example, oxygen atoms can combine
with hydrogen atoms on cold dust grains to form water, or a free oxygen
split from O2 by ultraviolet radiation can recombine with an O2 molecule to form ozone (O3).

- Despite its detection on the icy
moons of Jupiter and Saturn, O2 had been missing in the inventory of
volatile species associated with comets until now. “We
weren’t really expecting to detect O2 at the comet
– and in such high abundance – because it is so chemically
reactive, so it was quite a surprise,” says Kathrin Altwegg of
the University of Bern, and principal investigator of the ROSINA-DFMS
(Rosetta Orbiter Spectrometer for Ion and Neutral Analysis-Double
Focusing Mass Spectrometer) instrument.

- The research team analyzed more
than 3000 samples collected around the comet between September 2014 and
March 2015 to identify the O2. They determined an abundance of 1–10% relative to H2O,
with an average value of 3.80 ± 0.85%, an order of magnitude
higher than predicted by models describing the chemistry in molecular
clouds. The amount of molecular oxygen detected showed a strong
relationship to the amount of water measured at any given time,
suggesting that their origin on the nucleus and release mechanism are
linked. By contrast, the amount of O2 seen was poorly correlated with carbon monoxide and molecular nitrogen, even though they have a similar volatility to O2. In addition, no ozone was detected.

Figure 96:
Rosetta has made the first detection of molecular oxygen at a comet.
The results presented in this graphic are based on ROSINA-DFMS
observations between September 2014 and March 2015 when Rosetta was
still on the approach to the Sun along its orbit (image credit:
Spacecraft: ESA/ATG medialab; comet: ESA/Rosetta/NavCam – CC
BY-SA IGO 3.0; Data: A. Bieler et al. (2015)

•
October 2015 (Rosetta navigation during lander delivery phase and
reconstruction of Philae descent trajectory and rebound): 154)155)
The first three months around the comet were dedicated to its
characterization and to the identification of candidate landing sites
for the Philae lander. On November 12th, Philae separated from Rosetta,
starting its 7-hour descent to the target landing site: Agilkia.
Philae’s telemetry, received on ground via Rosetta, confirmed
that landing occurred at 15:34:04 (UTC, on-board time), but also that
Philae did not succeed to anchor to the comet surface. This caused the
lander to bounce and continue an additional 2-hour flight, in which it
collided with a crater rim; it had another smaller bounce; until it
finally landed on the comet surface, where it successfully executed its
primary science mission. In this phase, Rosetta navigation was very
demanding in terms of operations workload and required navigation
accuracy. It is considered a success, since all operations could be
conducted nominally and the first landing point was well within the a
priori landing error ellipse.

- Rosetta’s SPD (Lander
Delivery Phase) started on October 28, 2014 with the execution of the
SDP-1 maneuver that drove the spacecraft away from the “Close
Observation” 10 km orbit, in a transition arc to the 30 km
parking orbit, in preparation for the lander delivery sequence to be
executed on November 12. The main objectives of this phase were: to
safely deliver Philae into its descent trajectory towards the target
landing site; to keep the communication link with the lander during
descent and immediately after landing; and additionally to take images
of the lander’s descent and landing.

- Trajectory: Figure 97
shows, in white, the terminator orbits flown by Rosetta the month
before lander delivery (from 20 km to 10 km distance); and in red the
trajectory flown by Rosetta for lander delivery:

Legend to Figure 97:
(1) Rosetta is inserted in a circular 30 km parking orbit with an
orbital plane tilted 15º from the terminator plane (plane
separating day/night). Rosetta flies almost a full orbit revolution
around the comet, half of it spent in the night side of the comet (2).
Once Rosetta arrives at the target point (3) for the initiation of the
lander delivery sequence, the pre-delivery maneuver (83.6 cm/s) is
executed, driving the S/C in a hyperbolic trajectory, almost in
collision course, with 5 km miss-distance. A mission constraint did not
allow to drive the orbiter in a collision trajectory to the comet, even
if the plan included a subsequent maneuver that would avoid the
collision.

At a distance of 22.6 km (4),
Philae separates from Rosetta. The nominal separation’s relative
velocity was 18.76 cm/s. Taking into account the mass of Rosetta and
Philae and the fraction of Rosetta’s propellant intervening in
the separation, it results that, nominally, the lander would receive a
ΔV of 17.4 cm/s and Rosetta of 1.1 cm/s. The ΔV received by
the lander redirected its trajectory towards the target point on the
comet’s surface. The direction in which the separation is
performed fixes the orientation of the lander’s Z-axis, which is
stabilized by the angular momentum stored in its flywheel. The descent
trajectory was designed such that at touchdown (7 hours after
separation) the lander’s Z-axis was parallel to the local
vertical (defined as the normal to the local surface), and that the
incoming relative velocity with respect to the surface was also
parallel to the local vertical.

In the meantime, Rosetta executes
the post-delivery maneuver to avoid passing at 5 km distance from the
comet, staying in the day side of the comet and ensuring communications
with Philae during descent. Philae’s antenna is mounted pointing
along the lander’s +Z axis and, by design, it is not pointing to
Rosetta after separation. Therefore, a communication link during
descent would not be possible if Rosetta did not execute the
post-delivery maneuver. Afterwards, the mission enters in the Relay
Phase (5), in which Rosetta performs a series of maneuvers to keep its
trajectory in the region where the lander visibilities durations are
maximized, in order to support the scientific operations of the lander
on the comet’s surface.

- Navigation
analysis: In the scope of the design of the lander delivery trajectory,
navigation analysis was performed to assess the achievable landing
accuracy. Monte Carlo simulations were run reproducing the navigation
process: randomly generating a “real-world” trajectory;
based on it, simulated observations are generated and fed to the OD
(Orbit Determination); from the OD solution, the landing trajectory is
optimized; and finally, the resulting maneuvers and separation
conditions are perturbed and applied to the real-world to propagate the
trajectory and compute the achieved landing point.

- The results of this analysis
showed that the landing uncertainty was covering an area of roughly 500
m radius around the target landing point and that the uncertainty in
the landing time was of about 40 min. Figure 98 shows the landing points of several simulated trajectories plotted on top of a NAVCAM image. 156)

- On Nov. 12, 2014, Rosetta released
Philae. The small lander, after 7 h descent, finally touched softly the
ground of comet Churyumov-Gerasimenko. Unfortunately its anchoring
system failed and Philae experienced a 2 h bouncing trajectory. It
landed 1 km away from its target site. Nevertheless it was operated
during 57 h performing its FSS (First Science Sequence. The FSS, made
possible with the two batteries, should have been followed by the LTS
(Long Term Science Sequence) but Philae was not well illuminated and
fell “asleep”. One of the last Philae actions was to rotate
its head slightly (+22° around Z axis) to improve solar arrays
illumination. 157)

The wake-up of
Philae occurred on June 13, 2015. A very short and unstable RF link was
established between Philae and Rosetta. The lander was still not
properly illuminated. The final position and attitude of Philae were
key data to forecast an eventual wake-up. But as Philae was not
equipped with system dedicated to position and attitude monitoring, the
only way to determine missing data was to examine all collected
measurements, housekeeping as well as scientific data. During the next
weeks, Rosetta and Philae teams tried to assess the Philae’s
status and to improve the RF links. Unfortunately, due to comet
outgassing, it was not possible to reduce the distance between the two
spacecrafts significantly. The last telecommunication occurred on July
9, 2015.

The Science Operation Navigation
Center was responsible to coordinate and to realize activities
necessary to determine the Philae attitude and position. Thanks to a
collective effort, a possible landing area and an attitude were
successfully estimated. Equivalent work was also realized to achieve
the reconstruction, both of trajectory and attitude, of the descent and
bouncing on the surface.

• Sept.28, 2015: Two comets
collided at low speed in the early Solar System to give rise to the
distinctive ‘rubber duck’ shape of Comet
67P/Churyumov–Gerasimenko, say Rosetta scientists. -The
origin of the comet’s double-lobed form has been a key question
since Rosetta first revealed its surprising shape in July 2014. Two
leading ideas emerged: did two comets merge or did localized erosion of
a single object form the ‘neck’? 158)

- Now, scientists have an
unambiguous answer to the conundrum. By using high-resolution images
taken between 6 August 2014 and 17 March 2015 to study the layers of
material seen all over the nucleus, they have shown that the shape
arose from a low-speed collision between two fully fledged, separately
formed comets.

- “It is clear from the images
that both lobes have an outer envelope of material organized in
distinct layers, and we think these extend for several hundred meters
below the surface,” says Matteo Massironi, lead author from the
University of Padova, Italy, and an associate scientist of the OSIRIS
team. “You can imagine the layering a bit like an onion, except
in this case we are considering two separate onions of differing size
that have grown independently before fusing together.”

- The results of the study are
reported in the journal Nature and were presented on Sept. 28, 2015 at
the European Planetary Science Congress in Nantes, France. 159)

Legend to Figure 99:
A selection of high-resolution OSIRIS images used to identify patterns
in Comet 67P/Churyumov–Gerasimenko’s extensive layering.

- Top left: main terraces
(green) and exposed layers (red dashed lines) seen in the Seth region
on the comet’s large lobe. The terraces become more inclined
towards the comet neck region. The close-up shows terraces in two
locations (thin white and yellow arrows) together with examples of
parallel lineaments (large white arrows) that define a continuous
stratification.

- Bottom left: outline of
exposed layers (red dashed lines) primarily in the Imhotep and Ash
region on the comet’s large lobe. The terraces in Ash change
their dip direction from that in Seth to very slightly dip towards
Imhotep. Some layers are also indicated on the comet’s small lobe
in the background. The close-up shows the details of the parallel
layers in a section along the Imhotep-Ash boundary.

- Top right: main layers (red
dashed lines) and cross-cutting fractures (blue dashed lines) in the
Hathor cliff face on the comet’s small lobe. No abrupt change in
the orientation of the layers is seen between Hathor and Ma’at.
The close-up shows stratification in an alcove at the Hathor-Anuket
boundary, providing a view of the Anuket inner structure, which appears
to extend under Ma’at. Terraces on Anuket (white arrows) are seen
in different orientations to neighboring regions. Taken together, this
reinforces the idea that Hathor represents the inner comet structure
that has been exposed, with Anuket as the remnant.

- Bottom right:
layers (white dashed lines) at the boundary of Anubis and Seth on the
comet’s large lobe. This continuous scarp suggests the thickness
of the Seth region is about 150 m. The three arrow heads point to a
terrace margin in Anubis and the single white arrow points to a terrace
in the adjacent Atum region.

To reach their conclusion, Matteo
and his colleagues first used images to identify over 100 terraces seen
on the surface of the comet, and parallel layers of material clearly
seen in exposed cliff walls and pits. A 3D shape model was then used to
determine the directions in which they were sloping and to visualize
how they extend into the subsurface.

It soon became clear that the
features were coherently oriented all around the comet’s lobes
and in some places extended to a depth of about 650 m. “This was
the first clue that the two lobes are independent, reinforced by the
observation that the layers are inclined in opposite directions close
to the comet’s neck,” says Matteo. “To be sure, we
also looked at the relationship between the local gravity and the
orientations of the individual features all around the reconstructed
comet surface.”

Broadly speaking, layers of
material should form at right angles to the gravity of an object. The
team used models to compute the strength and direction of the gravity
at the location of each layer. In one case, they modelled the comet as
a single body with a center of mass close to the neck. In the other,
they worked with two separate comets, each with its own center of mass.
The team found that orientation of a given layer and the direction of
the local gravity are closer to perpendicular in the model with two
separate objects, rather than in the one with a single combined nucleus
(Ref. 158).

Legend to Figure 100:
The methods used by Rosetta scientists to determine that Comet
67P/Churyumov–Gerasimenko’s shape arises from two
separately forming comets.

- Left: high-resolution OSIRIS
images were used to visually identify over 100 terraces (green) or
strata – parallel layers of material (red dashed lines) –
in exposed cliff walls and pits all over the comet surface (top: Hathor
and surrounding regions on comet’s small lobe; bottom: Seth
region on comet’s large lobe).

- Middle: a 3D shape model was used
to determine the directions in which the terraces/strata are sloping
and to visualize how they extend into the subsurface. The strata
‘planes’ are shown superimposed on the shape model (left
panel) and alone (right panel) and show the planes coherently oriented
all around the comet, in two separate bounding envelopes (scale bar
indicates angular deviation between plane and local gravity vector).

- Right: local gravity vectors
visualized on the comet shape model perpendicular to the terrace/strata
planes further realize the independent nature of the two lobes (Ref. 158).

• Sept.
23, 2015: Comets are celestial bodies comprising a mixture of dust and
ices, which they periodically shed as they swing towards their closest
point to the Sun along their highly eccentric orbits. As sunlight heats
the frozen nucleus of a comet, the ice in it – mainly water but
also other 'volatiles' such as carbon monoxide and carbon dioxide
– turns directly into a gas. This gas flows away from the comet,
carrying dust particles along. Together, gas and dust build up the
bright halo and tails that are characteristic of comets. 160)

- Rosetta arrived at Comet
67P/Churyumov–Gerasimenko in August 2014 and has been studying it
up close for over a year. On 13 August 2015, the comet reached the
closest point to the Sun along its 6.5-year orbit, and is now moving
back towards the outer Solar System.

- A key feature that Rosetta's
scientists are investigating is the way in which activity on the comet
and the associated outgassing are driven, by monitoring the increasing
activity on and around the comet since Rosetta's arrival.

- Scientists using Rosetta's
Visible, InfraRed and Thermal Imaging Spectrometer, VIRTIS, have
identified a region on the comet's surface where water ice appears and
disappears in sync with its rotation period (Figure 101).
Their findings are published in the journal Nature. The results are
based on images and spectra taken at visible and infrared wavelengths
of light on 12–14 September 2014 with VIRTIS. 161)

- The research team found a
mechanism that replenishes the surface of the comet with fresh ice at
every rotation: this keeps the comet 'alive'," says Maria Cristina De
Sanctis from INAF-IAPS in Rome, Italy. The team studied a set of data
taken in September 2014, concentrating on a 1 km2 region on
the comet's neck. At the time, the comet was about 500 million km from
the Sun and the neck was one of the most active areas. As the comet
rotates, taking just over 12 hours to complete a full revolution, the
various regions undergo different illumination. "We saw the tell-tale
signature of water ice in the spectra of the study region but only when
certain portions were cast in shadow," says Maria Cristina.
"Conversely, when the Sun was shining on these regions, the ice was
gone. This indicates a cyclical behavior of water ice during each comet
rotation."

- The data suggest that water ice on
and a few cm below the surface 'sublimates' when illuminated by
sunlight, turning it into gas that then flows away from the comet.
Then, as the comet rotates and the same region falls into darkness, the
surface rapidly cools again. However, the underlying layers remain warm
owing to the sunlight they received in the previous hours, and, as a
result, subsurface water ice keeps sublimating and finding its way to
the surface through the comet's porous interior. But as soon as this
'underground' water vapor reaches the cold surface, it freezes again,
blanketing that patch of comet surface with a thin layer of fresh ice.
- Eventually, as the Sun rises again over this part of the surface on
the next comet day, the molecules in the newly formed ice layer are the
first to sublimate and flow away from the comet, restarting the cycle.

- "We suspected such a water ice
cycle might be at play at comets, on the basis of theoretical models
and previous observations of other comets but now, thanks to Rosetta's
extensive monitoring at 67P/Churyumov–Gerasimenko, we finally
have observational proof," says Fabrizio Capaccioni, VIRTIS principal
investigator at INAF-IAPS in Rome, Italy. From these data, it is
possible to estimate the relative abundance of water ice with respect
to other material. Down to a few cm deep over the region of the portion
of the comet nucleus that was surveyed, water ice accounts for
10–15% of the material and appears to be well-mixed with the
other constituents.

- The scientists also calculated how
much water vapor was being emitted by the patch that they analyzed with
VIRTIS, and showed that this accounted for about 3% of the total amount
of water vapor coming out from the whole comet at the same time, as
measured by Rosetta's MIRO microwave sensor. "It is possible that many
patches across the surface were undergoing the same diurnal cycle, thus
providing additional contributions to the overall outgassing of the
comet," adds Fabrizio Capaccioni. The scientists are now busy analyzing
VIRTIS data collected in the following months, as the comet's activity
increased around the closest approach to the Sun.

- The images were taken on 12 (top),
13 (middle) and 14 September (bottom) and focus on Hapi, a region on
the comet's 'neck', one of the most active spots on the nucleus at the
time.

- By comparing these images and
maps, the scientists have found that water ice is present on colder
patches, while it is less abundant or absent on warmer patches. In
addition, water ice was only detected on a patch of the surface when it
was cast in shadow. This indicates a cyclical behavior of water ice
during each comet rotation.

- Right: the daily water ice cycle.
During the local day, water ice on and a few centimeters below the
surface sublimates and escapes; during the local night, the surface
rapidly cools while the underlying layers are still warm, so subsurface
water ice continues sublimating and finding its way to the surface,
where it freezes again. On the next comet day, sublimation starts
again, beginning from water ice in the newly formed surface layer.

• Sept.
15, 2015: New results from PTOLEMY – the OU (Open University) led
instrument on the Rosetta mission’s Philae lander, suggest that
Comet 67P/Churyumov-Gerasimenko may be giving of different gases from
different parts of its surface, making it heterogeneous in nature.
PTOLEMY – the gas analysis instrument on board Philae, has taken
measurements of the concentration of volatile molecules at the
lander’s final resting site, Abydos. Its findings have shown the
presence of both water (H2O) and carbon dioxide (CO2),
but of very little carbon monoxide (CO). These findings follow the
first set of results published by the Ptolemy team last month which
reported the presence of organic compounds in the surface dust on Comet
67P. 163)164)

- The Ptolemy team have been
surprised by the results as, based on the findings of the ROSINA
instrument on board the Rosetta orbiter, they were expecting to see
larger concentrations of CO on the surface. ROSINA, like PTOLEMY, is a
mass spectrometer and at the time of landing was analyzing the gases
rising from the surface some 30 km above Comet 67P. Results by ROSINA,
acquired shortly before landing (published in January 2015), found that
the concentration of CO, although variable, was up to four times that
of CO2, whereas the PTOLEMY measurements found that CO was about ten times less than CO2.

- According to Andrew Morse, lead
author on the paper, these findings could suggest that either the coma
gas composition changes through various chemical reactions as it moves
away from the comet, or that the gas vaporised from the comet varies by
location, making it a heterogeneous comet. He says: “Though it is
a possibility that carbon monoxide is produced in the coma as it moves
away from the comet, a more probable account of such a large change
would be that the gases released from the comet’s surface differs
according to location.”

- One hypothesis is that a
heterogeneous comet is the result of it being accumulated from diverse
building blocks during its formation in the solar system. Alternatively
it is the result of uneven heating in its journey into the inner solar
system. The ROSINA instrument could help to answer this by making
further measurements of the water coming off the comet’s surface.
Andrew Morse adds: “Our results, from Comet 67P’s surface,
has both surprised us as well as opened up a variety of new questions
about how comets form and how they work. We’re eagerly awaiting
new results which should help us to clarify whether Comet 67P is indeed
heterogeneous in nature or if there is another explanation. Either way,
these results offer up an important piece of the complex, yet
fascinating puzzle of how comets are formed.”

- Co-author Geraint Morgan says:
“The questions raised by PTOLEMY show the value of landers, even
high risk ones, in establishing a ‘ground truth’
measurement on the surface to compare with on-going measurements from
orbiting spacecraft. Despite Philae’s eventful journey, the data
produced could be the key to help the Rosetta mission unlock the
secrets of the Solar System.”

• August 13: 2015: ESA’s
Rosetta today witnessed Comet 67P/Churyumov–Gerasimenko making
its closest approach to the Sun. The exact moment of perihelion
occurred at 02:03 GMT this morning when the comet came within 186
million km of the Sun. 165)

- In the year that has passed since
Rosetta arrived, the comet has travelled some 750 million km along its
orbit towards the Sun, the increasing solar radiation heating up the
nucleus and causing its frozen ices to escape as gas and stream out
into space at an ever greater rate. These gases, and the dust particles
that they drag along, build up the comet’s atmosphere –
coma – and tail.

- The activity reaches its peak
intensity around perihelion and in the weeks that follow – and is
clearly visible in the spectacular images returned by the spacecraft in
the last months. One image taken by Rosetta’s navigation camera
was acquired at 01:04 GMT, just an hour before the moment of
perihelion, from a distance of around 327 km.

- The scientific camera is also
taking images today – the most recent available image was taken
at 23:31 GMT on 12 August, just a few hours before perihelion. The
comet’s activity is clearly seen in the images, with a multitude
of jets stemming from the nucleus, including one outburst captured in
an image taken at 17:35 GMT yesterday.

Figure 102:
This series of images of Comet 67P/Churyumov–Gerasimenko was
captured by Rosetta’s OSIRIS narrow-angle camera on 12 August
2015, just a few hours before the comet reached the closest point to
the Sun along its 6.5-year orbit, or perihelion (image credit:
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Legend to Figure 102:
The image at left was taken at 14:07 GMT, the middle image at 17:35
GMT, and the final image at 23:31 GMT. The images were taken from a
distance of about 330 km from the comet. The comet’s activity, at
its peak intensity around perihelion and in the weeks that follow, is
clearly visible in these spectacular images. In particular, a
significant outburst can be seen in the image captured at 17:35 GMT.

- “Activity will remain high
like this for many weeks, and we’re certainly looking forward to
seeing how many more jets and outburst events we catch in the act, as
we have already witnessed in the last few weeks,” says Nicolas
Altobelli, acting Rosetta project scientist.

- Rosetta’s measurements
suggest the comet is spewing up to 300 kg of water vapor –
roughly the equivalent of two bathtubs – every second. This is a
thousand times more than was observed this time last year when Rosetta
first approached the comet. Then, it recorded an outflow rate of just
300 g per second, equivalent to two small glasses of water.

- Along with gas, the nucleus is
also estimated to be shedding up to 1000 kg of dust per second,
creating dangerous working conditions for Rosetta.

Rosetta Continued

• August 11, 2015: In the
approach to perihelion over the past few weeks, Rosetta has been
witnessing growing activity from Comet 67P/Churyumov–Gerasimenko,
with one dramatic outburst event proving so powerful that it even
pushed away the incoming solar wind . 166)

- The period around perihelion is
scientifically very important, as the intensity of the sunlight
increases and parts of the comet previously cast in years of darkness
are flooded with sunlight. Although the comet’s general activity
is expected to peak in the weeks following perihelion, much as the
hottest days of summer usually come after the longest days, sudden and
unpredictable outbursts can occur at any time – as already seen
earlier in the mission.

- On 29 July, Rosetta observed the
most dramatic outburst yet, registered by several of its instruments
from their vantage point 186 km from the comet. They imaged the
outburst erupting from the nucleus, witnessed a change in the structure
and composition of the gaseous coma environment surrounding Rosetta,
and detected increased levels of dust impacts (Figure 104).
Perhaps most surprisingly, Rosetta found that the outburst had pushed
away the solar wind magnetic field from around the nucleus.

- A sequence of images taken by
Rosetta’s scientific camera OSIRIS show the sudden onset of a
well-defined jet-like feature emerging from the side of the
comet’s neck, in the Anuket region. It was first seen in an image
taken at 13:24 GMT, but not in an image taken 18 minutes earlier, and
has faded significantly in an image captured 18 minutes later. The
camera team estimates the material in the jet to be travelling at 10
m/s at least, and perhaps much faster. “This is the brightest jet
we’ve seen so far,” comments Carsten Güttler, OSIRIS
team member at the MPS (Max Planck Institute for Solar System Research)
in Göttingen, Germany.

- The decrease
in magnetic field strength was measured by Rosetta’s RPC-MAG
instrument during the outburst event on 29 July 2015. This is the first
time a ‘diamagnetic cavity’ has been detected at Comet
67P/Churyumov–Gerasimenko and is thought to be caused by an
outburst of gas temporarily increasing the gas flux in the
comet’s coma, and pushing the pressure-balance boundary between
it and incoming solar wind farther from the nucleus than expected under
‘normal’ levels of activity (Figure 103).

Figure 104:
Rosetta’s scientific camera OSIRIS shows the sudden onset of a
well-defined jet-like feature emerging from the side of the
comet’s neck, in the Anuket region on July 29, 2015 (image
credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

- Soon afterwards, the comet
pressure sensor of ROSINA detected clear indications of changes in the
structure of the coma, while its mass spectrometer recorded changes in
the composition of outpouring gases. For example, compared to
measurements made two days earlier, the amount of carbon dioxide
increased by a factor of two, methane by four, and hydrogen sulphide by
seven, while the amount of water stayed almost constant.

- During an outburst of gas and dust
from Comet 67P/Churyumov–Gerasimenko on 29 July 2015,
Rosetta’s ROSINA instrument detected a change in the composition
of gases compared with previous days. Figure 105
shows the relative abundances of various gases after the outburst,
compared with the measurements two days earlier. For example, the
amount of carbon dioxide (CO2) increased by a factor of two, methane (CH4) by four, and hydrogen sulphide (H2S) by seven, while the amount of water (indicated by the horizontal black line) stayed almost constant.

• August
10, 2015: What a difference a year can make. Rosetta arrived at Comet
67P/Churyumov–Gerasimenko on 6 August 2014, achieving rendezvous
at a distance of 100 km before moving even closer to the nucleus in the
following weeks. The image shown on the left was taken with the
navigation camera, NAVCAM, on rendezvous day, when Rosetta was about
121 km out. 167)

Figure 106: Images of Comet
67P/Churyumov–Gerasimenko on 6 August 2014 (left) and on 6 August
2015 (right) with increased exposure to the Sun’s energy and its
resulting activity (image credit: ESA/Rosetta/NAVCAM – CC BY-SA
IGO 3.0)

• On August 6, 2015,
ESA’s Rosetta mission celebrates one year at Comet
67P/Churyumov–Gerasimenko, with its closest approach to the Sun
now just one week away. 168)

- It’s been a long but
exciting journey for Rosetta since its launch in 2004, featuring Earth,
Mars and two asteroid flybys before arriving at its ultimate
destination on 6 August 2014. Over the following months, the mission
became the first ever to orbit a comet and the first to soft land a
probe – Philae – on its surface.

• July
30, 2015: Complex molecules that could be key building blocks of life,
the daily rise and fall of temperature, and an assessment of the
surface properties and internal structure of the comet are just some of
the highlights of the first scientific analysis of the data returned by
Rosetta's lander Philae last November. 169)

- Early results from Philae's first
suite of scientific observations of Comet 67P/Churyumov-Gerasimenko
were published today in a special edition of the journal Science
(Science Special Issue, July 31, 2015). 170)

- Data were obtained during the
lander's seven-hour descent to its first touchdown at the Agilkia
landing site, which then triggered the start of a sequence of
predefined experiments. But shortly after touchdown, it became apparent
that Philae had rebounded and so a number of measurements were carried
out as the lander took flight for an additional two hours some 100 m
above the comet, before finally landing at Abydos.

- Meanwhile, Ptolemy sampled ambient
gas entering tubes at the top of the lander and detected the main
components of coma gases – water vapor, carbon monoxide and
carbon dioxide, along with smaller amounts of carbon-bearing organic
compounds, including formaldehyde.

- A timeline of the science
operations that Rosetta's lander Philae performed between 12 and 15
November 2014, following touchdown on the surface of Comet
67P/Churyumov–Gerasimenko. This is an update on the original
first science sequence. Following Philae's unexpected flight across the
surface of the comet, the planned first science sequence had to be
adapted according to the new situation. The graphic shows the
approximate times (to the nearest 15 minutes) that each of Philae's 10
data acquired. 171)

• July 20, 2015: On 9 July
2015 at 19:45 CEST, Philae reported back to the team at DLR/LCC (Lander
Control Center) only to then go back to 'silent mode'. Since then, the
team has been working hard to get back in contact with the lander and
operate it to conduct scientific measurements. "We sent a command to
turn on the ROMAP (Rosetta Lander Magnetometer and Plasma Monitor), but
have not seen a response," explains DLR's Philae project leader Stephan
Ulamec. Using an identical model in the MUSC (Microgravity User Support
Center) at DLR, the engineers are currently testing various commands,
with which they want to enable and optimize Philae. "In the telemetry
received, we have observed signs that Philae could have moved and that
its antennas are thus perhaps more concealed or their orientation might
have changed." 172)

- Philae's move: In the data
previously sent by Philae from the surface of Comet
67P/Churyumov-Gerasimenko about its condition, the lander has also
transmitted information about the sunlight reaching its solar panels.
“This profile – where panels are receiving a great deal of
sunlight – has clearly changed between June and July,” says
Ulamec. “This cannot be explained only by the course of the
seasons on the comet.” The lander could have moved, for example,
due to outgassing during the comet's awakening. After a not entirely
smooth landing on 12 November 2014, Philae finally halted at a crater
rim on uneven terrain – for this reason, even a slight change in
its position could mean that its antennas are now obstructed by more
objects above it. This would affect communication with Philae.

- Blind commands as backup: It is
also possible that one of the lander's two radio receiver units is
damaged and that one of the transmitter units is not fully functional.
However, Philae is programmed to switch back and forth between the two
transmitters periodically. This could also explain why contact with
Philae is irregular. "We have therefore tested a command on our ground
model that will cause Philae to only interact with the functional
transmitter." This command has been transmitted to the lander. This
'blind commanding' – without the lander sending a confirmation
– should make it possible for it to receive the command and
execute it as soon as it is supplied with solar energy during the comet
day and switches on.

- The engineers at the LCC are also
testing another command on the ground model of Philae; they want to try
to activate a 'work package' on the lander that was successfully
executed in November 2014 during the landing and is still stored by
Philae. At that time, the team at the LCC had supplied the lander with
a kind of 'emergency program', so that it could still operate five
instruments without communication. This occurred as the engineers at
the consoles had to adjust their plans to adapt to the evolving
situation with a new landing site. "With this work package, the thermal
probe MUPUS measured temperatures, ROMAP and SESAME conducted
measurements, and PTOLEMY and COSAC researched in 'sniff' mode," says
Ulamec. "All of these instruments require no detailed commands, but the
stored work package must first of all be retrieved." If this idea
works, once Philae switches on, it would start to conduct scientific
measurements and then send the data to Earth.

- Interaction
between lander and orbiter: Until 24 July 2015, the Rosetta orbiter
will fly an orbit that satisfies the requirements of the lander and
follow a path that is favorable for communication between the two
spacecraft. Then, Rosetta will fly with its 11 instruments over the
southern hemisphere of the comet, which is now increasingly illuminated
by the Sun. Here, the attempts to communicate with Philae will
alternate with the priorities for observation with the orbiter
instruments. The comet's increasing activity – with its gas and
dust ejections –does not allow the orbiter to fly very close to
the comet's surface. On 12 July 2015, Rosetta’s star trackers
were once again affected by the dusty environment. For this reason, the
orbiter is now flying at a safer distance of 170–190 km.

- Of course, the Philae lander team
at DLR has not given up. "The lander is obviously still functional,
because it sends us data, albeit at irregular intervals and at
surprising times," says Ulamec. "There have been several times when we
feared that the lander would not switch back on, but it has repeatedly
taught us otherwise."

• July 13, 2015:
Rosetta’s investigations of its comet are continuing as the
mission teams count down the last month to perihelion – the
closest point to the Sun along the comet’s orbit – when the
comet’s activity is expected to be at its
highest.“Perihelion is an important milestone in any
comet’s calendar, and even more so for the Rosetta mission
because this will be the first time a spacecraft has been following a
comet from close quarters as it moves through this phase of its journey
around the Solar System,” notes Matt Taylor, ESA’s Rosetta
project scientist. “We’re looking forward to reaching
perihelion, after which we’ll be continuing to monitor how the
comet’s nucleus, activity and plasma environment changes in the
year after, as part of our long-term studies.” 173)

Figure 109: The orbit of Comet
67P/Churyumov-Gerasimenko and its approximate location around
perihelion, the closest the comet gets to the Sun. The positions of the
planets are correct for 13 August 2015 (image credit: ESA)

• July 10, 2015: The Philae
lander communicated with the Rosetta orbiter again between 19:45 and
20:07 CEST (Central European Standard Time) on 9 July 2015 and
transmitted measurement data from the CONSERT (COmet Nucleus Sounding
Experiment by Radiowave Transmission) instrument. Although the
connection failed repeatedly after that, it remained completely stable
for those 12 minutes. "This sign of life from Philae proves to us that
at least one the lander's communication units remains operational and
receives out commands," said German Aerospace Center (Deutsches Zentrum
für Luft- und Raumfahrt; DLR) engineer Koen Geurts, a member of
the lander control team at DLR Cologne. The mood had been mixed over
the last few days; Philae had not communicated with the team in the
DLR/LCC (Lander Control Center) since 24 June 2015. After an initial
test command to turn on the power to CONSERT on 5 July 2015, the lander
did not respond. Philae’s team began to wonder if the lander had
survived on Comet 67P/Churyumov-Gerasimenko. 174)

- Commanded
from the ground successfully: "We never gave up on Philae and remained
optimistic," said Geurts. There was great excitement when Philae
'reported in' on 13 June 2015 after seven months of hibernation and
sent data about its health. The lander was ready to perform its tasks,
300 million km away from Earth. However, Philae has to communicate with
the ground stations through Rosetta, which acts as a radio relay.
Restrictions on the orbiter's approach, orbiting around the comet, have
not permitted regular communication with the lander. The data sent on
24 June did not suggest that the lander had experienced technical
difficulties. Now, Philae's internal temperature of 0ºC gives the
team hope that the lander can charge its batteries; this would make
scientific work possible regardless of the 'time of day' on the comet.

- Currently, DLR's lander team is
evaluating the data that were received. "We can already see that the
CONSERT instrument was successfully activated by the command we sent on
9 July," explained Geurts. Even now, Philae is causing the team some
puzzlement: "We do not yet have an explanation for why the lander has
communicated now, but not over the past few days.” The trajectory
of the orbiter, for example, has not changed over the last three weeks.
However, one thing is certain; Philae has survived the harsh conditions
on the comet and is responding to commands from the LCC team. "This is
extremely good news for us," said Geurts.

• July 1, 2015: A number of
the dust jets emerging from Rosetta’s comet can be traced back to
active pits that were likely formed by a sudden collapse of the
surface. These ‘sinkholes’ are providing a glimpse at the
chaotic and diverse interior of the comet. 175)

- In a study reported in the science
journal Nature, 18 quasi-circular pits have been identified in the
northern hemisphere of the comet, some of which are the source of
continuing activity. The pits are a few tens to a few hundreds of
meters in diameter and extend up to 210 m below the surface to a smooth
dust-covered floor. Material is seen to be streaming from the most
active pits. The study team observes jets arising from the fractured
areas of the walls inside the pits. These fractures mean that volatiles
trapped under the surface can be warmed more easily and subsequently
escape into space, according to Jean-Baptiste Vincent from the Max
Planck Institute for Solar System Research, lead author of the study. 176)

Left: 18 pits have been identified
in high-resolution OSIRIS images of Comet
67P/Churyumov–Gerasimenko’s northern hemisphere. The pits
are named after the region they are found in, and some of them are
active. The context image was taken on 3 August 2014 by the
narrow-angle camera from a distance of 285 km; the image resolution is
5.3 m/pixel.

Middle, top: close-up of the active
pit named Seth_01 reveals small jets emanating from the interior walls
of the pit. The close-up also shows the complex internal structure of
the comet. The image is a section of an OSIRIS wide-angle camera image
capture on 20 October 2014 from a distance of 7 km from the comet
surface. Seth_01 measures about 220 m across.

Right, top: context image showing
fine structure in the comet’s jets as seen from a distance of 28
km from the comet’s surface on 22 November 2014. The image was
taken with the OSIRIS wide-angle camera and has a resolution is 2.8
m/pixel. In both images the contrast is deliberately stretched in order
to see the details of the activity. The active pits in this study
contribute a small fraction of the observed activity.

Left, bottom:
how the pits may form through sinkhole collapse. 1. Heat causes
subsurface ices to sublimate (blue arrows), forming a cavity (2). When
the ceiling becomes too weak to support its own weight, it collapses,
creating a deep, circular pit (3, red arrow). Newly exposed material in
the pit walls sublimates, accounting for the observed activity (3, blue
arrows).

• June 26. 2015: Despite a new
trajectory for Rosetta and a reduction of the distance between the
orbiter and Comet 67P/Churyumov-Gerasimenko from 200 to 180 km, contact
with the Philae lander remains irregular and short. After the initial
contact on 13 June 2015, Philae has reported to the DLR/LCC (Lander
Control Center) in Cologne a total of six times. However, for the last
three possibilities calculated for establishing a connection with
Philae, no data could be received. "Right now, we are playing with the
geometry between the Rosetta orbiter and the Philae lander," says DLR's
Philae Project Manager, Stephan Ulamec. "The most recent contact
– on 24 June 2015 – lasted 20 minutes; then, the line went
dead again." Now, the DLR and ESA mission teams are analyzing which
measures will make better contact with Philae possible. 178)

- On June 27, ESA will begin new
maneuvers, which will move the Rosetta orbiter 20 km closer to the
comet's surface and Philae by 30 June 2015. The team at the DLR control
center hopes that contact with Philae at a distance of 160 km will then
be regular and stable. The next few days will show whether changes in
the geometry between the lander and orbiter improves communication with
Philae.

- One possible reason for the
lander's current silence could be a failure of Philae's communications
equipment caused by poor conditions during hibernation. Analysis of the
data received so far by the team at DLR has shown that while one of the
communications units is compromised, the other unit has worked thus far
without problems. "To continue conducting scientific work with Philae,
we rely on long and predictable contact times," says Ulamec. Once
Philae can receive and execute extensive command sequences safely,
store the measurement data and send it to the ground team, its 10
instruments will be operated again.

• June 24, 2015: Exposed water ice detected on Comet's surface. Using
the high-resolution science OSIRIS camera on board ESA’s Rosetta
spacecraft, scientists have identified more than a hundred patches of
water ice a few meters in size on the surface of Comet
67P/Churyumov-Gerasimenko. A new study just published in the journal
Astronomy & Astrophysics focuses on an analysis of bright patches
of exposed ice on the comet’s surface. 179)180)

- Based on observations of the gas
emerging from comets, they are known to be rich in ices. As they move
closer to the Sun along their orbits, their surfaces are warmed and the
ices sublimate into gas, which streams away from the nucleus, dragging
along dust particles embedded in the ice to form the coma and tail.

- But some of the comet’s dust
also remains on the surface as the ice below sublimates, or falls back
on to the nucleus elsewhere, coating it with a thin layer of dusty
material and leaving very little ice directly exposed on the surface.
These processes help to explain why Comet 67P/Churyumov-Gerasimenko and
other comets seen in previous flyby missions are so dark.

- Despite this, Rosetta’s
suite of instruments has already detected a variety of gases, including
water vapor, carbon dioxide and carbon monoxide, thought to originate
from frozen reservoirs below the surface.

- Now, using images taken with
Rosetta’s OSIRIS narrow-angle camera last September, scientists
have identified 120 regions on the surface of Comet
67P/Churyumov-Gerasimenko that are up to ten times brighter than the
average surface brightness. Some of these bright features are found in
clusters, while others appear isolated, and when observed at high
resolution, many of them appear to be boulders displaying bright
patches on their surfaces.

- The clusters of bright features,
comprising a few tens of meter-sized boulders spread over several tens
of meters, are typically found in debris fields at the base of cliffs.
They are most likely the result of recent erosion or collapse of the
cliff wall revealing fresher material from below the dust-covered
surface.

- By contrast, some of the isolated
bright objects are found in regions without any apparent relation to
the surrounding terrain. These are thought to be objects lifted up from
elsewhere on the comet during a period of cometary activity, but with
insufficient velocity to escape the gravitational pull of the comet
completely.

- In all cases, however, the bright
patches were found in areas that receive relatively little solar
energy, such as in the shadow of a cliff, and no significant changes
were observed between images taken over a period of about a month.
Furthermore, they were found to be bluer in color at visible
wavelengths compared with the redder background, consistent with an icy
component.

- “Water ice is the most
plausible explanation for the occurrence and properties of these
features,” says Antoine Pommerol of the University of Bern and
lead author of the study. “At the time of our observations, the
comet was far enough from the Sun such that the rate at which water ice
would sublimate would have been less than 1 mm per hour of incident
solar energy. By contrast, if carbon dioxide or carbon monoxide ice had
been exposed, it would have rapidly sublimated when illuminated by the
same amount of sunlight. Thus we would not expect to see that type of
ice stable on the surface at this time.”

- The team
also turned to laboratory experiments that tested the behavior of water
ice mixed with different minerals under simulated solar illumination in
order to gain more insights into the process. They found that after a
few hours of sublimation, a dark dust mantle a few millimeters thick
was formed. In some places this acted to completely conceal any visible
traces of the ice below, but occasionally larger dust grains or chunks
would lift from the surface and move elsewhere, exposing bright patches
of water ice. “A 1 mm thick layer of dark dust is sufficient to
hide the layers below from optical instruments,” confirms Holger
Sierks, OSIRIS principal investigator at the Max Planck Institute for
Solar System Research in Göttingen.

- The team also speculates about the
timing of the formation of the icy patches. One hypothesis is that they
were formed at the time of the last closest approach of the comet to
the Sun, 6.5 years ago, with icy blocks ejected into permanently
shadowed regions, preserving them for several years below the peak
temperature needed for sublimation.

- Another idea is that even at
relatively large distances from the Sun, carbon dioxide and carbon
monoxide driven-activity could eject the icy blocks. In this scenario,
it is assumed that the temperature was not yet high enough for water
sublimation, such that the water-ice-rich components outlive any
exposed carbon dioxide or carbon monoxide ice.

- “As the comet continues to
approach perihelion, the increase in solar illumination onto the bright
patches that were once in shadow should cause changes in their
appearance, and we may expect to see new and even larger regions of
exposed ice,” says Matt Taylor, ESA’s Rosetta project
scientist. “Combining OSIRIS observations made pre- and
post-perihelion with other instruments will provide valuable insight
into what drives the formation and evolution of such regions.”

Figure 111: Ice on Comet 67P/Churyumov-Gerasimenko

• June 23, 2015: The adventure continues: ESA confirmed that its Rosetta mission will be extended until the end of September 2016,
at which point the spacecraft will most likely be landed on the surface
of Comet 67P/Churyumov-Gerasimenko.Rosetta’s nominal mission was
originally funded until the end of December 2015, but at a meeting
today, ESA’s Science Program Committee has given formal approval
to continue the mission for an additional nine months. At that point,
as the comet moves far away from the Sun again, there will no longer be
enough solar power to run Rosetta’s set of scientific
instrumentation efficiently. 181)

- “This is fantastic news for
science,” says Matt Taylor, ESA’s Rosetta Project
Scientist. “We’ll be able to monitor the decline in the
comet’s activity as we move away from the Sun again, and
we’ll have the opportunity to fly closer to the comet to continue
collecting more unique data. By comparing detailed ‘before and
after’ data, we’ll have a much better understanding of how
comets evolve during their lifetimes.”

- Comet
67P/Churyumov-Gerasimenko will make its closest approach to the Sun on
13 August and Rosetta has been watching its activity increase over the
last year. Continuing its study of the comet in the year following
perihelion will give scientists a fuller picture of how a comet’s
activity waxes and wanes along its orbit.

- As the activity diminishes
post-perihelion, it should be possible to move the orbiter much closer
to the comet’s nucleus again, to make a detailed survey of
changes in the comet’s properties during its brief
‘summer’. In addition, there may be an opportunity to make
a definitive visual identification of Philae. Although candidates have
been seen in images acquired from a distance of 20 km, images taken
from 10 km or less after perihelion could provide the most compelling
confirmation.

- During the extended mission, the
team will use the experience gained in operating Rosetta in the
challenging cometary environment to carry out some new and potentially
slightly riskier investigations, including flights across the
night-side of the comet to observe the plasma, dust, and gas
interactions in this region, and to collect dust samples ejected close
to the nucleus.

- As the comet recedes from the Sun,
the solar-powered spacecraft will no longer receive enough sunlight to
operate efficiently and safely, equivalent to the situation in June
2011 when the spacecraft was put into hibernation for 31 months for the
most distant leg of its journey out towards the orbit of Jupiter.

Figure 112: A Rosetta NAVCAM
camera single frame image of Comet 67P/Churyumov-Gerasimenko, acquired
on 5 June 2015 from a distance of 208 km from the comet center. The
image has a resolution of 17.7 m/pixel and measures 18.1 km across
(image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

• June 19, 2015: The DLR team
received data from the Philae lander for the third time on 19 June
2015. Between 13:20 and 13:39 UTC, Philae sent 185 data packets. "Among
other things, we have received updated status information," says
Michael Maibaum, a systems engineer at the DLR (Lander Control Center)
in Cologne and Deputy Operations Manager. "At present, the lander is
operating at a temperature of zero degrees Celsius, which means that
the battery is now warm enough to store energy. This suggests that
Philae will also be able to work during the comet's night, regardless
of solar illumination." In the 19 minutes of transmission, the lander
sent data recorded last week; from this, the engineers determined that
the amount of sunlight has increased: "More solar panels were
illuminated; at the end of contact, four of Philae's panels were
receiving energy". There were a number of interruptions in the
connection, but it was otherwise stable over a longer period for the
first time. “The contact has confirmed that Philae is doing very
well.” 182)

• June 15, 2015: The receipt
of signals from Rosetta's Philae lander on 13 June after 211 days of
hibernation marked the start of intense activity. In coordination with
its mission partners, ESA teams are working to juggle Rosetta's flight
plan to help with renewed lander science investigations. 183)

- Since March 2015, when Philae's
environmental conditions started to improve with higher surface
temperatures and better illumination, the orbiter's receiver had been
turned on periodically to listen for signals from the lander when the
orbital geometry was thought to be optimum. On the evening of 13 June,
a weak but solid radio link between Rosetta and the lander was finally
established for 85 seconds. More than 300 'packets' (663 kbit) of
lander housekeeping telemetry were received. This information had been
stored on board at an as-yet-to-be determined time in the past, as much
as several days to a few weeks, so does not necessarily reflect the
lander's current status.

- "We are still examining the
housekeeping information at the Lander Control Center in the DLR German
Aerospace Center's establishment in Cologne, but we can already tell
that all lander subsystems are working nominally, with no apparent
degradation after more than half a year hiding out on the comet's
frozen surface," says DLR's Stephan Ulamec, Philae Lander Project
Manager.

- A second, smaller burst of lander
data was received on Sunday, 14 June, at about 21:26 UTC, lasting just
a few seconds. These data were confirmed to give the current status,
showing the lander's internal temperature had already risen to
–5ºC.

- Engineers at the Lander Control
Center have determined that Philae is already being exposed to
sufficient sunlight to heat it to an acceptable operating temperature
and to generate electricity.

- "Power
levels increase during the local 'comet day' – the part of the
about-12 hour comet rotation when Philae is in sunlight – from 13
W at comet sunrise to above 24 W," notes ESA's Patrick Martin, Rosetta
Mission Manager. "It needs at least 19 W to switch on the transmitter."

- The telemetry downloaded covered
the lander's status for a full night–day cycle of the comet,
which is helping ground teams to understand how the Sun is shining on
the lander. The solar panels appear to be receiving power for over 135
minutes in each illumination period.

- The main task now for all the
mission partners — ESA for Rosetta operations and DLR and
France's CNES space agency for lander operations and science,
respectively — is to determine how to optimize Rosetta's orbit so
as to facilitate contact and enable new science investigations. It is
believed that there is sufficient power now being generated to allow
some science measurements during the time Philae is illuminated, with
initial activities focusing on low-power measurements. This first phase
would also likely include measurements that did not previously generate
science in November.

- However, the mission teams first
must establish a more robust link between Rosetta and Philae before
uploading the first batch of science operations commands. The quality
of the communication link is also possibly related to the trajectory
Rosetta is flying and the orientation it adopts.

- Optimizing an orbit 305 million km away:
Currently, Rosetta experiences two possible communication slots per 24
hours – once per 12-hour comet rotation. Until 23:35 UTC on
Tuesday, 16 June, Rosetta will be flying an orbit set by
already-uploaded commands on the terminator – the plane between
comet day and night – moving out from about 200 km to 235 km
altitude.

- This orbit is not optimized for
lander communication, so longer periods of contact may not be possible
until the trajectory has been changed. "With work done by the flight
dynamics and operations team at ESOC and based on intense planning
being conducted with the mission partners today, a new orbit will be
devised that ensures optimum lander communications beginning with the
next command upload later tonight," says Paolo Ferri, ESA's Head of
Mission Operations.

- This new orbit will include an
already-planned reduction of distance from the nucleus, down to 180 km
versus 200 km, and 'nadir pointing' – continuously pointing
Rosetta's communications unit at the comet. In the coming days, the
orbiter may also be moved closer to the comet, without compromising the
safety of the spacecraft, to help communications.

- The new orbit will be flown by
Rosetta starting after 23:25 UTC on 16 June until 19 June, aiming to
enable more and longer contacts with Philae, especially towards the end
of this period. Establishing a regular and predictable pattern of
contacts is a prerequisite for performing a complete assessment of the
lander's status and for planning science operations. "If we manage to
achieve and maintain a predictable contact pattern," continues Paolo
Ferri, "the lander teams can devise a strategy for a new sequence of
scientific operations.

- As a bonus, any operation of
Philae's instruments up to or through perihelion on 13 August –
the comet's closest point to the Sun along its orbit – will allow
in-situ study of a comet during its peak activity.

• The Philae lander has
reported back on 13 June 2015 at 20:28 (UTC), coming out of hibernation
and sending the first data to Earth. More than 300 data packets
have been analyzed by the team at the DLR ( German Aerospace Center)
Lander Control Center: "Philae is doing very well – it has an
operating temperature of minus 35º Celsius and has 24 W of power
available," explains DLR’s Philae Project Manager, Stephan
Ulamec. "The lander is ready for operations." Philae 'spoke' for 85
seconds with its team on ground in its first contact since it went into
hibernation. The signals were also received at ESA/ESOC in Darmstadt,
Germany. 184)185)

- When analyzing the status data, it
became clear that Philae also must have been awake earlier: "We have
also received historical data – until now, however, the lander
had not been able to contact us. "Now, the scientists are waiting for
the next contact. In Philae's mass memory, there are still more than
8000 data packets, which will give the DLR team information on what
happened to Philae in the past few days on comet Churyumov-Gerasimenko.

- Philae shut down on 15 November
2014 at 02:15 UTC, after being in operation on the comet for about 60
hours. Since 12 March 2015, the communication unit on the Rosetta
orbiter has repeatedly been turned on to communicate with the lander
and receive its reply.

•
June 2, 2015: Rosetta's continued close study of Comet
67P/Churyumov–Gerasimenko has revealed an unexpected process at
work, causing the rapid breakup of water and carbon dioxide molecules
spewing from the comet’sn surface. - One instrument, the Alice
spectrograph provided by NASA, has been examining the chemical
composition of the comet's atmosphere, or coma, at far-ultraviolet
wavelengths. At these wavelengths, Alice allows scientists to detect
some of the most abundant elements in the Universe such as hydrogen,
oxygen, carbon and nitrogen. The spectrograph splits the comet's light
into its various colors – its spectrum – from which
scientists can identify the chemical composition of the coma gases. 186)187)

- In a study,
scientists from several institutions report the detections made by
Alice from Rosetta’s first four months at the comet, when the
spacecraft was between 10 km and 80 km from the center of the comet
nucleus. The research team focused on the nature of 'plumes' of water
and carbon dioxide gas erupting from the comet's surface, triggered by
the warmth of the Sun. To do so, they looked at the emission from
hydrogen and oxygen atoms resulting from broken water molecules, and
similarly carbon atoms from carbon dioxide molecules, close to the
comet nucleus. They discovered that the molecules seem to be broken up
in a two-step process. 188)

- First, an ultraviolet photon from
the Sun hits a water molecule in the comet’s coma and ionizes it,
knocking out an energetic electron. This electron then hits another
water molecule in the coma, breaking it apart into two hydrogen atoms
and one oxygen, and energizing them in the process. These atoms then
emit ultraviolet light that is detected at characteristic wavelengths
by Alice.

- Similarly, it is the impact of an
electron with a carbon dioxide molecule that results in its break-up
into atoms and the observed carbon emissions. "Analysis of the relative
intensities of observed atomic emissions allows us to determine that we
are directly observing the ‘parent’ molecules that are
being broken up by electrons in the immediate vicinity, about 1 km, of
the comet’s nucleus where they are being produced," says Paul
Feldman, professor of physics and astronomy at the Johns Hopkins
University in Baltimore, and lead author of the paper discussing the
results.

- By comparison, from Earth or from
Earth-orbiting space observatories such as the Hubble Space Telescope,
the atomic constituents of comets can only be seen after their parent
molecules, such as water and carbon dioxide, have been broken up by
sunlight, hundreds to thousands of kilometers away from the nucleus of
the comet. "The discovery we’re reporting is quite unexpected,"
says Alice Principal Investigator Alan Stern, an associate
vice-president in the Space Science and Engineering Division of the
Southwest Research Institute (SwRI). "It shows us the value of going to
comets to observe them up close, since this discovery simply could not
have been made from Earth or Earth orbit with any existing or planned
observatory. And, it is fundamentally transforming our knowledge of
comets."

- The results from Alice are
supported by data obtained by other Rosetta instruments, in particular
MIRO, ROSINA and VIRTIS, which are able to study the abundance of
different coma constituents and their variation over time, and particle
detecting instruments like RPC-IES. "These early results from Alice
demonstrate how important it is to study a comet at different
wavelengths and with different techniques, in order to probe various
aspects of the comet environment," says ESA’s Rosetta project
scientist Matt Taylor. "We’re actively watching how the comet
evolves as it moves closer to the Sun along its orbit towards
perihelion in August, seeing how the plumes become more active due to
solar heating, and studying the effects of the comet’s
interaction with the solar wind."

• April 14, 2015: Measurements
made by Rosetta and Philae during the probe’s multiple landings
on Comet 67P/Churyumov-Gerasimenko show that the comet’s nucleus
is not magnetized. 189)190)

- Studying the properties of a comet
can provide clues to the role that magnetic fields played in the
formation of Solar System bodies almost 4.6 billion years ago. The
infant Solar System was once nothing more than a swirling disc of gas
and dust but, within a few million years, the Sun burst into life in
the center of this turbulent disc, with the leftover material going
into forming the asteroids, comets, moons and planets. — The dust
contained an appreciable fraction of iron, some of it in the form of
magnetite. Indeed, millimeter-sized grains of magnetic materials have
been found in meteorites, indicating their presence in the early Solar
System.

- This leads scientists to believe
that magnetic fields threading through the proto-planetary disc could
have played an important role in moving material around as it started
to clump together to form larger bodies. But it remains unclear as to
how crucial magnetic fields were later on in this accretion process, as
the building blocks grew to centimeters, meters and then tens of meters
across, before gravity started to dominate when they grew to hundreds
of meters and kilometers in scale. Some theories concerning the
aggregation of magnetic and non-magnetic dust particles show that the
resulting bigger objects could also remain magnetised, allowing them to
also be influenced by the magnetic fields of the proto-planetary disc.

- Because comets contain some of the
most pristine materials in the Solar System, they offer a natural
laboratory for investigating whether or not these larger chunks could
have remained magnetized. However, detecting the magnetic field of
comets has proven difficult in previous missions, which have typically
made rapid flybys, relatively far from comet nuclei. — It has
taken the proximity of ESA’s Rosetta orbiter to Comet
67P/Churyumov-Gerasimenko, and the measurements made much closer to and
at the surface by its lander Philae, to provide the first detailed
investigation of the magnetic properties of a comet nucleus.

• Reconstructing Philae’s trajectory:
Magnetic field data from ROMAP on Philae, combined with information
from the CONSERT experiment that provided an estimate of the final
landing region, timing information, images from Rosetta’s OSIRIS
camera, assumptions about the gravity of the comet, and measurements of
its shape, have been used to reconstruct the trajectory of the lander
during its descent and subsequent landings on and bounces over the
surface of Comet 67P/Churyumov-Gerasimenko on 12 November 2014. The
times are as recorded by the spacecraft; the confirmation signals
arrived on Earth 28 minutes later. 191)

- Initially, Philae was seen to
rotate slowly during the descent to Agilkia. It landed and then
bounced, rotating significantly faster as the momentum of the internal
flywheel was transferred to the lander. It collided with a cliff 45
minutes later, then tumbled, flying above the surface for more than an
hour longer, before bouncing once again and coming to a stop a few
meters away, a few minutes later.

- The position of the first
touchdown point at Agilkia is very well determined from direct images,
but the locations of the possible cliff collision depends on the
ballistic model used, while the general location marked for the
subsequent second and third touchdowns at Abydos come from the CONSERT
measurements. Thus, these latter positions represent preliminary and
approximate locations only. - The heights above the surface assume a
reference sphere centered on the center of mass of the comet and with a
radius of 2393 m reaching first touchdown point.

- Philae’s magnetic field
measuring instrument is the ROMAP (Rosetta Lander Magnetometer and
Plasma Monitor), while Rosetta carries a magnetometer as part of the
RPC-MAP (Rosetta Plasma Consortium suite of sensors). Changes in the
magnetic field surrounding Rosetta allowed RPC-MAG to detect the moment
when Philae was deployed in the morning of 12 November 2014.

- Then, by sensing periodic
variations in the measured external magnetic field and motions in its
boom arm, ROMAP was able to detect the touchdown events and determine
the orientation of Philae over the following hours. Combined with
information from the CONSERT experiment that provided an estimate of
the final landing site location, timing information, images from
Rosetta’s OSIRIS camera, assumptions about the gravity of the
comet, and measurements of its shape, it was possible to determine
Philae’s trajectory.

- The mission teams soon discovered
that Philae not only touched down once at Agilkia, but also came into
contact with the comet’s surface four times in fact –
including a grazing collision with a surface feature that sent it
tumbling towards the final touchdown point at Abydos. This complex
trajectory turned out to be scientifically beneficial to the ROMAP
team.

- “The unplanned flight across
the surface actually meant we could collect precise magnetic field
measurements with Philae at the four points we made contact with, and
at a range of heights above the surface,” says Hans-Ulrich
Auster, co-principal investigator of ROMAP and lead author of the
results published in the journal Science and presented at the European
Geosciences Union General Assembly in Vienna, Austria (April 12-17,
2015). 192)

- The multiple descents and ascents
meant that the team could compare measurements made on the inward and
outward journeys to and from each contact point, and as it flew across
the surface. ROMAP measured a magnetic field during these sequences,
but found that its strength did not depend on the height or location of
Philae above the surface. This is not consistent with the nucleus
itself being responsible for that field.

- “If the surface was
magnetized, we would have expected to see a clear increase in the
magnetic field readings as we got closer and closer to the
surface,” explains Hans-Ulrich. “But this was not the case
at any of the locations we visited, so we conclude that Comet
67P/Churyumov-Gerasimenko is a remarkably non-magnetic object.”

- Instead, the magnetic field that
was measured was consistent with an external one, namely the influence
of the solar wind interplanetary magnetic field near the comet nucleus.
This conclusion is confirmed by the fact that variations in the field
that were measured by Philae closely agree with those seen at the same
time by Rosetta.

- “During Philae’s
landing, Rosetta was about 17 km above the surface, and we could
provide complementary magnetic field readings that rule out any local
magnetic anomalies in the comet’s surface materials,” says
Karl-Heinz Glassmeier, principal investigator of RPC-MAG on board the
orbiter and a co-author of the Science paper.

- If large chunks of material on the
surface of 67P/Churyumov-Gerasimenko were magnetized, ROMAP would have
recorded additional variations in its signal as Philae flew over them.
“If any material is magnetized, it must be on a scale of less
than one meter, below the spatial resolution of our measurements. And
if Comet 67P/Churyumov-Gerasimenko is representative of all cometary
nuclei, then we suggest that magnetic forces are unlikely to have
played a role in the accumulation of planetary building blocks greater
than one meter in size,” concludes Hans-Ulrich.

- “It’s great to see the
complementary nature of Rosetta and Philae’s measurements,
working together to answer this simple, but important
‘yes-no’ question as to whether the comet is
magnetized,” says Matt Taylor, ESA’s Rosetta project
scientist.

Legend to Figure 116:
Magnetic field data collected by Philae’s ROMAP instrument
immediately before (top) and after (bottom) the cliff collision at
16:20 GMT on 12 November 2014 (onboard spacecraft time), between the
first and second touchdowns. Height above the surface is plotted on the
x-axis and magnetic field strength on the y-axis. Therefore time runs
left-to-right for the ascent (lower) plot, but right-to-left for the
descent (upper) plot.

The measurements (crosses) are
compared with a hypothetical model (solid line) assuming a slightly
magnetized surface. Also included is the strength of and variation in
the external field, namely the influence of the solar wind
interplanetary magnetic field near the comet nucleus.

At distances of 10 m or greater
from the surface, the surface component would be very weak, leaving
just the external field, as measured. But closer to the surface, the
comet’s own field should increase and dominate. That is not seen,
therefore the data suggest that at scales of greater than one meter
(the resolution of the instrument), the comet is not magnetized.

• April 13, 2015: Four months
from today, on 13 August, Comet 67P/Churyumov-Gerasimenko will reach
perihelion – a moment that defines its closest point to the Sun
along its orbit. For 67P/Churyumov-Gerasimenko, this takes place at a
distance of about 185 million km from the Sun, between the orbits of
Earth and Mars. The Rosetta spacecraft is along for the ride, and has
been watching the gradual evolution of the comet since arriving in
August 2014. 193)

- As the comet’s surface
layers are gently warmed, frozen ices sublimate. The escaping gas
carries streams of dust out into space, and together these slowly
expand to create the comet’s fuzzy atmosphere, or coma. —
As the comet continues to move closer to the Sun, the warming continues
and activity rises, and pressure from the solar wind causes some of the
materials to stream out into long tails, one made of gas, the other of
dust. The comet’s coma will eventually span tens of thousands of
kilometers, while the tails may extend hundreds of thousands of
kilometers, and both will be visible through large telescopes on Earth.

- But it is Rosetta’s close
study of the comet, from just a few tens of kilometers above its
surface, which enables the source of the comet’s activity to be
studied in great detail, providing context to the more distant
ground-based observations.

- This spectacular montage of 18 images (Figure 117)
shows off the comet’s activity from many different angles as seen
between 31 January (top left) and 25 March (bottom right), when the
spacecraft was at distances of about 30 to 100 km from the comet. At
the same time, Comet 67P/Churyumov-Gerasimenko was at distances between
363 million and 300 million km from the Sun. — After perihelion,
Rosetta will continue to follow the comet, watching how the activity
subsides as it moves away from the Sun and back to the outer Solar
System again.

- This leads scientists to believe
that magnetic fields threading through the proto-planetary disc could
have played an important role in moving material around as it started
to clump together to form larger bodies.

• March 20, 2015: Perhaps it
is still too cold for the Philae lander to wake up on Comet
67P/Churyumov-Gerasimenko. Maybe its power resources are not yet
sufficient to send a signal to the team at DLR (German Aerospace
Center) Lander Control Center. On 12 March 2015, the Rosetta orbiter
began to send signals to the lander and listen for a response, but
Philae has not yet reported back. "It was a very early attempt; we will
repeat this process until we receive a response from Philae," says DLR
Project Manager Stephan Ulamec. "We have to be patient." On 20 March
2015 at 05:00 CET (Central European Time), the communication unit on
the Rosetta orbiter was switched off. Now, the DLR team is calculating
when the next favorable alignment between the orbiter and the lander
will occur, and will then listen again for a signal from Philae. The
next chance to receive a signal from the lander is expected to occur
during the first half of April. 194)

• March 19, 2015: ESA’s
Rosetta spacecraft has made the first measurement of molecular nitrogen
at a comet, providing clues about the temperature environment in which
Comet 67P/Churyumov–Gerasimenko formed. The in situ detection of
molecular nitrogen has long been sought at a comet. Nitrogen had only
previously been detected bound up in other compounds, including
hydrogen cyanide and ammonia, for example. 195)

- Its detection is particularly
important since molecular nitrogen is thought to have been the most
common type of nitrogen available when the Solar System was forming. In
the colder outer regions, it likely provided the main source of
nitrogen that was incorporated into the gas planets. It also dominates
the dense atmosphere of Saturn’s moon, Titan, and is present in
the atmospheres and surface ices on Pluto and Neptune’s moon
Triton.

Legend to Figure 118: The graph shows the variation in the signals measured for molecular nitrogen (N2)
and carbon monoxide (CO) by Rosetta’s ROSINA instrument. The
signals vary as a function of time, comet rotation and position of the
spacecraft above the comet. An average ratio of N2/CO = (5.70 ± 0.66) x 10–3 was determined for the period 17–23 October 2014. The minimum and maximum values measured were 1.7 x 10–3 and 1.6 x 10–2,
respectively (note that the ratio cannot be derived directly from this
graph – a correction factor accounting for the instrument
sensitivity is applied).

By comparing the ratio of N2
to CO at the comet with that of the protosolar nebula, it was
determined that the comet must have formed at low temperatures,
consistent with a Kuiper Belt origin. The study also finds that
Jupiter-family comets like Comet 67P/ Churyumov–Gerasimenko were
unlikely the source of Earth’s nitrogen.

• On March 4, 2015, ESA posted the four NAVCAM images of Figure 119
from distances of 80-100 km, to show the current activity of the comet.
While most of Rosetta’s NAVCAM images are taken for navigation
purposes, these images were obtained to provide context in support of
observations performed at the same time with the Alice ultraviolet (UV)
imaging spectrograph on Rosetta. The four images show the nucleus at
different orientations, providing a good overview of the comet’s
activity over the time interval between 25 and 27 February 2015. 196)

Figure 119:
Montage of four single-frame images of Comet 67P/C-G taken by
Rosetta’s Navigation Camera (NAVCAM) on Feb.25 (top left), Feb.
26, (top right) and the two bottom pictures on Feb. 27, 2015. The
images have been processed to bring out the details of the
comet’s activity. The exposure time for each image is 2 s (image
credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0)

• March 3, 2015: Images from
the OSIRIS scientific imaging camera taken during the close flyby on
February 14, 2015 have now been downlinked to Earth, revealing the
surface of Comet 67P/C-G in unprecedented detail, and including the
shadow of the spacecraft encircled in a wreath of light. During the
flyby, Rosetta not only passed closer by the comet than ever before,
but also passed through a unique observational geometry: for a short
time the Sun, spacecraft, and comet were exactly aligned. In this
geometry, surface structures cast almost no shadows, and therefore the
reflection properties of the surface material can be discerned. 197)

Legend to Figure 120:
Close-up view of a 228 x 228 m region on Comet
67P/Churyumov-Gerasimenko, as seen by the OSIRIS narrow-angle camera
during Rosetta’s flyby at 12:39 UTC on 14 February 2015. The
image was taken 6 km above the comet’s surface, and the image
resolution is just 11 cm/pixel. Rosetta’s fuzzy shadow, measuring
approximately 20 x 50 m, is seen at the bottom of the image.

• Feb. 17, 2015: The
spacecraft is no longer orbiting the comet, it is now performing a
series of flybys to continue its science. 198)

- The video of Ref. 198)
explains the next stage of the Rosetta mission, the science that will
be done during 2015 by the orbiter’s flybys, and assesses the
possibility of the Philae lander’s reactivation from hibernation.
So far Rosetta has only mapped about seventy percent of the surface
because the comet’s orbit and rotation kept certain areas in
darkness. — This year new regions will come into view alongside
new activity on the surface. When the comet is at the peak of its
activity in the summer, Rosetta’s instruments will be there to
observe, measure and record a spectacular event.

• On 14 February 2015, Rosetta
swooped over the surface of Comet 67P/Churyumov–Gerasimenko at a
distance of just 6 km. The closest approach took place at 12:41 GMT
over a region known as Imhotep, which is on the larger of the
comet’s two lobes. 199)

- The image of Figure 121
reveals the contrasting terrains seen on this comet. Layered and
fractured exposed surfaces contrast against expanses of smooth,
dust-covered terrain. In some places, such as to the lower right of
this image, the faint outline of raised near-circular objects with
smooth surfaces can be seen. Elsewhere, boulders ranging in size from a
few meters to a few tens of meters are scattered across the surface.
The largest boulder, seen to the upper right, is named Cheops.

• January 26, 2015: ESA's
Rosetta mission is providing unique insight into the life cycle of a
comet's dusty surface, watching 67P/Churyumov-Gerasimenko as it sheds
the dusty coat it has accumulated over the past four years. 200)

- The COSIMA (COmetary Secondary Ion
Mass Analyzer) instrument is one of Rosetta's three dust analysis
experiments. It started collecting, imaging and measuring the
composition of dust particles shortly after the spacecraft arrived at
the comet in August 2014.

- Results from the first analysis of
its data are reported in the journal Nature on January 26, 2015. The
study covers August to October, when the comet moved along its orbit
between about 535 million km to 450 million km from the Sun. Rosetta
spent most of this time orbiting the comet at distances of 30 km or
less. 201)

Legend to Figure 122:
Two examples of dust grains were collected by the COSIMA instrument in
the period 25-31 October 2014. Both grains were collected at a distance
of 10-20 km from the comet nucleus. Image (a) shows a dust
particle,named by the COSIMA team as Eloi, that crumbled into a rubble
pile when collected; (b) shows a dust particle that shattered, named
Arvid.

For both grains, the image is shown
twice under two different grazing illumination conditions: the top
image is illuminated from the right, the bottom image from the left.
The brightness is adjusted to emphasize the shadows, in order to
determine the height of the dust grain. Eloi therefore reaches about
0.1 mm above the target plate; Arvid about 0.06 mm. The two small
grains at the far right of image (b) are not part of the shattered
cluster.

The fact that
the grains broke apart so easily means their individual parts are not
well glued together. If they contained ice they would not shatter;
instead, the icy component would evaporate off the grain shortly after
touching the collecting plate, leaving voids in what remained. By
comparison, if a pure water-ice grain had struck the detector, then
only a dark patch would have been seen.

These 'fluffy' grains are thought
to originate from the dusty layer built up on the comet's surface since
its last close approach to the Sun, and will soon be lost into the
coma.

- The scientists looked at the way
that many large dust grains broke apart when they were collected on the
instrument's target plate, typically at low speeds of 1-10 m/s. The
grains, which were originally at least 0.05 mm across, fragmented or
shattered upon collection. — The fact that they broke apart so
easily means that the individual parts were not well bound together.
Moreover, if they had contained ice, they would not have shattered.
Instead, the icy component would have evaporated off the grain shortly
after touching the collecting plate, leaving voids in what remained.

• January 22, 2015: Rosetta is
revealing its host comet as having a remarkable array of surface
features and with many processes contributing to its activity, painting
a complex picture of its evolution. In a special edition of the journal
Science, initial results are presented from seven of Rosetta's 11
science instruments based on measurements made during the approach to
and soon after arriving at Comet 67P/Churyumov–Gerasimenko in
August 2014. 202)

- The familiar shape of the
dual-lobed comet has now had many of its vital statistics measured: the
small lobe measures 2.6 x 2.3 x 1.8 km and the large lobe 4.1 x 3.3 x
1.8 km. The total volume of the comet is 21.4 km3 and the Radio Science Instrument has measured its mass to be 10 billion tons, yielding a density of 470 kg/m3.

- By assuming an overall composition dominated by water ice and dust with a density of 1500-2000 kg/m3,
the Rosetta scientists show that the comet has a very high porosity of
70-80%, with the interior structure likely comprising weakly bonded
ice-dust clumps with small void spaces between them.

Legend to Figure 123:
The 19 regions identified on Comet 67P/Churyumov–Gerasimenko are
separated by distinct geomorphological boundaries. Following the
ancient Egyptian theme of the Rosetta mission, they are named for
Egyptian deities. They are grouped according to the type of terrain
dominant within each region. Five basic categories of terrain type have
been determined: dust-covered (Ma’at, Ash and Babi); brittle
materials with pits and circular structures (Seth); large-scale
depressions (Hatmehit, Nut and Aten); smooth terrains (Hapi, Imhotep
and Anubis), and exposed, more consolidated (‘rock-like’)
surfaces (Maftet, Bastet, Serqet, Hathor, Anuket, Khepry, Aker, Atum
and Apis).

- The OSIRIS scientific camera, has
imaged some 70% of the surface to date: the remaining unseen area lies
in the southern hemisphere that has not yet been fully illuminated
since Rosetta’s arrival.

- Much of the northern hemisphere is
covered in dust. As the comet is heated, ice turns directly into gas
that escapes to form the atmosphere or coma. Dust is dragged along with
the gas at slower speeds, and particles that are not travelling fast
enough to overcome the weak gravity fall back to the surface instead.

Figure
124: Summary of the comet's vital statistics as determined by
Rosetta’s instruments during the first few months of its comet
encounter (image credit: ESA)

• Dec. 10, 2014: ESA’s
Rosetta spacecraft has found the water vapor from its target comet to
be significantly different to that found on Earth. The discovery fuels
the debate on the origin of our planet’s oceans. The
measurements were made in the month following the spacecraft’s
arrival at Comet 67P/Churyumov–Gerasimenko on 6 August. It is one
of the most anticipated early results of the mission, because the
origin of Earth’s water is still an open question. 203)204)205)206)

- One of the leading hypotheses on
Earth’s formation is that it was so hot when it formed 4.6
billion years ago that any original water content should have boiled
off. But, today, two thirds of the surface is covered in water, so
where did it come from? In this scenario, it should have been delivered
after our planet had cooled down, most likely from collisions with
comets and asteroids. The relative contribution of each class of object
to our planet’s water supply is, however, still debated.

- The key to determining where the
water originated is in its ‘flavor’, in this case the
proportion of deuterium – a form of hydrogen with an additional
neutron – to normal hydrogen. This proportion is an important
indicator of the formation and early evolution of the Solar System,
with theoretical simulations showing that it should change with
distance from the Sun and with time in the first few million years.

- One key goal is to compare the
value for different kinds of object with that measured for
Earth’s oceans, in order to determine how much each type of
object may have contributed to Earth’s water.

- Comets in particular are unique
tools for probing the early Solar System: they harbor material left
over from the protoplanetary disc out of which the planets formed, and
therefore should reflect the primordial composition of their places of
origin. - But thanks to the dynamics of the early Solar System, this is
not a straightforward process. Long-period comets that hail from the
distant Oort cloud originally formed in Uranus–Neptune region,
far enough from the Sun that water ice could survive. They were later
scattered to the Solar System’s far outer reaches as a result of
gravitational interactions with the gas giant planets as they settled
in their orbits.

- Conversely, Jupiter-family comets
like Rosetta’s comet were thought to have formed further out, in
the Kuiper Belt beyond Neptune. Occasionally these bodies are disrupted
from this location and sent towards the inner Solar System, where their
orbits become controlled by the gravitational influence of Jupiter.

- Indeed, Rosetta’s comet now
travels around the Sun between the orbits of Earth and Mars at its
closest and just beyond Jupiter at its furthest, with a period of about
6.5 years.

Legend to Figure 125:
Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H)
measured in the water vapor around Comet
67P/Churyumov–Gerasimenko. The measurements were made using
ROSINA’s DFMS double focusing mass spectrometer between 8 August
and 5 September 2014.

Deuterium is an isotope of hydrogen
with an added neutron. The ratio of deuterium to hydrogen in water is a
key diagnostic to determining where in the Solar System an object
originated and in what proportion asteroids and/or comets contributed
to Earth’s oceans.

Figure 125
displays the different values of D/H in water observed in various
bodies in the Solar System. The data points are grouped by color as
planets and moons (blue), chondritic meteorites from the Asteroid Belt
(grey), comets originating from the Oort cloud (purple) and Jupiter
family comets (pink). Rosetta’s Jupiter-family comet is
highlighted in yellow. Diamonds represent data obtained in situ;
circles represent data obtained by astronomical methods. The lower part
of the graph shows the value of D/H measured in molecular hydrogen in
the atmosphere of the giant planets of the Solar System (Jupiter,
Saturn, Uranus, Neptune) and an estimate of the typical value in
molecular hydrogen for the protosolar nebula, from which all objects in
our Solar System formed.

The ratio for Earth’s oceans is 1.56 x10–4
(shown as the blue horizontal line in the upper part of the graph). The
value for Comet 67P/Churyumov–Gerasimenko is found to be 5.3 x 10–4,
more than three times greater than for Earth’s oceans. The
discovery fuels the debate on the origin of Earth’s oceans and
whether asteroids or comets played the bigger role in delivering water.

Previous measurements of the
deuterium/hydrogen (D/H) ratio in other comets have shown a wide range
of values. Of the 11 comets for which measurements have been made, it
is only the Jupiter-family Comet 103P/Hartley 2 that was found to match
the composition of Earth’s water, in observations made by
ESA’s Herschel mission in 2011.

By contrast, meteorites
originally hailing from asteroids in the Asteroid Belt also match the
composition of Earth’s water. Thus, despite the fact that
asteroids have a much lower overall water content, impacts by a large
number of them could still have resulted in Earth’s oceans.

It is against this backdrop
that Rosetta’s investigations are important. Interestingly, the
D/H ratio measured by the Rosetta Orbiter Spectrometer for Ion and
Neutral Analysis, or ROSINA, is more than three times greater than for
Earth’s oceans and for its Jupiter-family companion, Comet
Hartley 2. Indeed, it is even higher than measured for any Oort cloud
comet as well.

“This surprising
finding could indicate a diverse origin for the Jupiter-family comets
– perhaps they formed over a wider range of distances in the
young Solar System than we previously thought,” says Kathrin
Altwegg, principal investigator for ROSINA. “Our finding also
rules out the idea that Jupiter-family comets contain solely Earth
ocean-like water, and adds weight to models that place more emphasis on
asteroids as the main delivery mechanism for Earth’s
oceans.”

“We knew that
Rosetta’s in situ analysis of this comet was always going to
throw up surprises for the bigger picture of Solar System science, and
this outstanding observation certainly adds fuel to the debate about
the origin of Earth’s water,” says Matt Taylor, ESA’s
Rosetta project scientist. “As Rosetta continues to follow the
comet on its orbit around the Sun throughout next year, we’ll be
keeping a close watch on how it evolves and behaves, which will give us
unique insight into the mysterious world of comets and their
contribution to our understanding of the evolution of the Solar
System.”

Table 17: Some background on previous measurements of the D/H ratio (Ref. 204)

Legend to Figure 126:
Deuterium is an isotope of hydrogen with an added neutron. The ratio of
deuterium to hydrogen in water is a key diagnostic to determining where
in the Solar System an object originated and in what proportion
asteroids and/or comets contributed to Earth’s oceans.

The data points are grouped by
color as planets and moons (blue), chondritic meteorites from the
Asteroid Belt (grey), comets originating from the Oort cloud (purple)
and Jupiter family comets (pink). Rosetta’s Jupiter-family comet
is highlighted in yellow. Diamonds represent data obtained in situ;
circles represent data obtained by astronomical methods. The lower part
of the graph shows the value D/H measured in molecular hydrogen in the
atmosphere of the giant planets of the Solar System (Jupiter, Saturn,
Uranus, Neptune) and an estimate of the typical value in molecular
hydrogen for the protosolar nebula, from which all objects in our Solar
System formed.

The horizontal blue line shows the value of the ratio in Earth's oceans, which has been determined to be 1.56 x 10–4.
Rosetta’s ROSINA instrument measured the water vapor emanating
from Comet 67P/Churyumov–Gerasimenko and found it to be 5.3 x 10–4, more than three times greater than for Earth’s oceans.

The discovery fuels the debate on
the origin of Earth’s oceans and whether asteroids or comets
played the bigger role in delivering water.

Legend to Figure 127:
The illustration shows the two main reservoirs of comets in the Solar
System: the Kuiper Belt, at a distance of 30–50 astronomical
units (AU: the Earth–Sun distance) from the Sun, and the Oort
Cloud, which may extend up to 50 000–100 000 AU from the Sun.

Halley’s comet is thought to
originate from the Oort Cloud, while 67P/Churyumov–Gerasimenko,
the focus of ESA’s Rosetta mission, hails from the Kuiper Belt.
It is now in a 6.5-year orbit around the Sun between the orbits of
Earth and Mars at its closest and just beyond Jupiter at its furthest.

• Nov.
28, 2014: Data collected by ROMAP (Rosetta Lander Magnetometer and
Plasma Monitor) onboard Philae, is being used to help reconstruct the
trajectory of the lander to its final landing site on Comet
67P/Churyumov-Gerasimenko. The scientists have now been able to use
ROMAP data to reconstruct the chain of events that took place on 12
November as follows: 210)

- Separation was confirmed as a
decay in the magnetic field perturbation as the distance between Philae
and the orbiter increased; at this point the lander was spinning at a
rate of about 1 rotation per 5 minutes.

- The ROMAP boom was deployed
successfully and a magnetic field decay was measured corresponding to
the increased distance of the ROMAP sensor with respect to its original
position on the lander.

- During the seven-hour descent, all
measurements were nominal, and ROMAP recorded the first touchdown at
15:34:04 GMT spacecraft time (the signal arrived on Earth just over 28
minutes later, and was confirmed at 16:03 GMT).

- After the first touchdown, the
spin rate started increasing. As the lander bounced off the surface,
the control electronics of the flywheel were turned off and during the
following 40 minutes of flight, the flywheel transferred its angular
momentum to Philae. After this time, the lander was spinning at a rate
of about 1 rotation per 13 seconds.

- At 16:20 GMT spacecraft time the lander is thought to have collided with a surface feature, a crater rim, for example.

“It was not a touchdown like
the first one, because there was no signature of a vertical
deceleration due to a slight dipping of the magnetometer boom as
measured during the first and also the final touchdown,”
according to Hans-Ulrich Auster, the PI of ROMAP at the TU
Braunschweig. It is assumed that Philae probably touched a surface with
one leg only – perhaps grazing a crater rim – and after
that the lander was tumbling. No simple rotation about the
lander’s z-axis could be seen.

- Following this event, the main rotation period had decreased slightly to 1 rotation per 24 seconds.

- At 17:25:26 GMT, Philae touched
the surface again, initially with just one foot but then all three,
giving the characteristic touchdown signal.

- At 17:31:17 GMT, after travelling probably a few more meters, Philae found its final parking position on three feet.

Despite this bumpy start to its
life on the comet, all of Philae’s instruments were operated in
the following two days. — The search for Philae’s final
landing spot is still on, and the ROMAP data are being used with other
instrument data from both Philae and Rosetta to try to reconstruct the
lander’s full trajectory and to identify its current location.
Also, a comprehensive scientific analysis of the data from all
instruments is underway.

• Nov. 19, 2014: With the
Philae lander’s mission complete, Rosetta will now continue its
own extraordinary exploration, orbiting Comet
67P/Churymov–Gerasimenko during the coming year as the enigmatic
body arcs ever closer to our Sun. The Rosetta spacecraft is in
excellent condition, with all of its systems and instruments performing
as expected. 211)

- Rosetta will become the first
spacecraft to witness at close quarters the development of a
comet’s coma and the subsequent tail streaming for millions of
kilometers into space. Rosetta will then have to stay further from the
comet to avoid the coma affecting its orbit. - In addition, as the
comet nears the Sun, illumination on its surface is expected to
increase. This may provide sufficient sunlight for the DLR-operated
Philae lander, now in hibernation, to reactivate, although this is far
from certain.

- Early next year, Rosetta will be
switched into a mode that allows it to listen periodically for beacon
signals from the surface.

• Nov. 17, 2014: The mosaic series in Figure 128
show the breathtaking journey of Rosetta’s Philae lander as it
approached and then rebounded from its first touchdown on Comet
67P/Churyumov–Gerasimenko on November 12, 2014. The images were
taken with Rosetta’s OSIRIS narrow-angle camera when the
spacecraft was 17.5 km from the comet center, or roughly 15.5 km from
the surface. They have a resolution of 28 cm/pixel and the enlarged
insets are 17 m x 17 m. 212)213)214)

Legend to Figure 128:
The mosaic comprises a series of images captured by Rosetta’s
OSIRIS camera over a 30 minute period spanning the first touchdown. The
time of each of image is marked on the corresponding insets and is in
GMT. A comparison of the touchdown area shortly before and after first
contact with the surface is also provided.

From left to right, the images show
Philae descending towards and across the comet before touchdown. The
image taken after touchdown, at 15:43 GMT, confirms that the lander was
moving east, as first suggested by the data returned by the CONSERT
experiment, and at a speed of about 0.5 m/s.

The final
location of Philae is still not known, but after touching down and
bouncing again at 17:25 GMT, it reached there at 17:32 GMT. The imaging
team is confident that combining the CONSERT ranging data with OSIRIS
and NAVCAM images from the orbiter and images from near the surface and
on it from Philae’s ROLIS and CIVA cameras will soon reveal the
lander’s whereabouts (Ref. 212).

Before going into hibernation at
00:36 GMT ( 01:36 CET) on 15 November 2014, the Philae lander was able
to conduct some work using power supplied by its primary battery. With
its 10 instruments, the mini laboratory sniffed the atmosphere,
drilled, hammered and studied Comet 67P/ Churyumov-Gerasimenko while
over 500 million km from Earth (Ref. 213).

- After being out of communication
visibility with the lander since 09:58 GMT / 10:58 CET on Nov. 14
(Friday), Rosetta regained contact with Philae at 22:19 GMT /23:19 CET
last night (Nov. 14). The signal was initially intermittent, but
quickly stabilized and remained very good until 00:36 GMT / 01:36 CET
this morning (Nov. 15). — In this period, the lander returned all
of its housekeeping data, as well as science data from the targeted
instruments, including ROLIS, COSAC, Ptolemy, SD2 and CONSERT. This
completed the measurements planned for the final block of experiments
on the surface.

- In addition, the lander’s
body was lifted by about 4 cm and rotated about 35° in an attempt
to receive more solar energy. But as the last science data fed back to
Earth, Philae’s power rapidly depleted. — “It has
been a huge success, the whole team is delighted,” said Stephan
Ulamec, lander manager at the DLR German Aerospace Agency, who
monitored Philae’s progress from ESOC in Darmstadt, Germany, this
week. - “We still hope that at a later stage of the mission,
perhaps when we are nearer to the Sun, that we might have enough solar
illumination to wake up the lander and re-establish communication,
” added Stephan.

- From now on, no contact will be
possible unless sufficient sunlight falls on the solar panels to
generate enough power to wake it up. The possibility that this may
happen later in the mission was boosted when mission controllers sent
commands to rotate the lander’s main body with its fixed solar
panels. This should have exposed more panel area to sunlight.

- Meanwhile, the Rosetta orbiter has
been moving back into a 30 km orbit around the comet. t will return to
a 20 km orbit on Dec. 6, 2014 and continue its mission to study the
body in great detail as the comet becomes more active, en route to its
closest encounter with the Sun on August 13, 2015. Over the coming
months, Rosetta will start to fly in more distant ‘unbound’
orbits, while performing a series of daring flybys past the comet, some
within just 8 km of its center.

Data collected by the orbiter will
allow scientists to watch the short- and long-term changes that take
place on the comet, helping to answer some of the biggest and most
important questions regarding the history of our Solar System. How did
it form and evolve? How do comets work? What role did comets play in
the evolution of the planets, of water on the Earth, and perhaps even
of life on our home world.

“The data collected by
Philae and Rosetta is set to make this mission a game-changer in
cometary science,” says Matt Taylor, ESA’s Rosetta project
scientist. Fred Jansen, ESA’s Rosetta mission manager, says,
“At the end of this amazing rollercoaster week, we look back on a
successful first-ever soft-landing on a comet. This was a truly
historic moment for ESA and its partners. We now look forward to many
more months of exciting Rosetta science and possibly a return of Philae
from hibernation at some point in time.”

• Nov.14, 2014 (update
information): Although the Philae touchdown was confirmed at ESOC to
have occurred at 16:03 GMT/17:03 CET on 12 November, scientists, flight
dynamics specialists and engineers from all mission participants have
been studying the first data returned from the lander. These revealed
the astonishing conclusion that the lander did not just touch down on
Comet 67P/Churyumov–Gerasimenko once, but three times. 216)

- The harpoons did not fire and
Philae appeared to be rotating after the first touchdown, which
indicated that it had lifted from the surface again. Stephan Ulamec,
Philae manager at the DLR German Aerospace Center, reported that it
touched the surface at 15:34, 17:25 and 17:32 GMT (comet time –
it takes over 28 minutes for the signal to reach Earth, via Rosetta).
The information was provided by several of the scientific instruments,
including the ROMAP magnetic field analyzer, the MUPUS thermal mapper,
and the sensors in the landing gear that were pushed in on the first
impact.

- The first
touchdown was inside the predicted landing ellipse, confirmed using the
lander’s downwards-looking ROLIS descent camera in combination
with the orbiter’s OSIRIS images to match features.

- But then the lander lifted from
the surface again – for 1 hour 50 minutes. During that time, it
travelled about 1 km upward at a speed of 38 cm/s. It then made a
smaller second hop, travelling at about 3 cm/s, and landing in its
final resting place seven minutes later.

- The touchdown signal generated on
first touchdown induced the instruments to ‘think’ that
Philae had landed, triggering the next sequence of experiments. Now
those data are being used to interpret the bounces.

- The lander remains unanchored to
the surface at an as yet undetermined orientation. The science
instruments are running and are delivering images and data, helping the
team to learn more about the final landing site.

- The descent camera revealed that
the surface is covered by dust and debris ranging in size from mm to m.
Meanwhile, Philae’s CIVA camera returned a panoramic image that
on first impressions suggests the lander is close to a rocky wall, and
perhaps has one of its three feet in open space.

- After discussions as to whether to
activate those science instruments that may cause the position of
Philae to shift, MUPUS and APXS have both been deployed.

- The primary battery enabling the
core science goals of the lander may run out some time in the next 24
hours. As for the secondary battery, charged by solar panels on Philae,
with only 1.5 hours of sunlight available to the lander each day, there
is an impact on the energy budget to conduct science for a longer
period of time. The original landing site offered nearly seven hours of
illumination per 12.4 hour comet day.

• Nov. 13, 2014: Rosetta’s lander Philae is safely on the surface of Comet 67P/Churyumov-Gerasimenko (Figure 131).
One of the lander’s three feet can be seen in the foreground. The
image is a two-image mosaic of CIVA (Comet Nucleus Infrared and Visible
Analyzer), a group of six identical micro-cameras. 217)218)

- Philae landed nearly vertically on
its side with one leg up in outer space. Philae settled into its final
landing spot after a harrowing first bounce that sent it flying as high
as a kilometer above the comet’s surface. After hovering for two
hours, it landed a second time only to bounce back up again a short
distance – this time 3 cm. Seven minutes later it made its third
and final landing. Incredibly, the little craft still functions after
trampolining for hours!

- Despite its awkward stance, Philae
continues to do a surprising amount of good science. Scientists are
still hoping to come up with a solution to better orientate the lander.
Their time is probably limited. The craft landed in the shadow of a
cliff, blocking sunlight to the solar panels used to charge its
battery. Philae receives only 1.5 hours instead of the planned 6-7
hours of sunlight each day.

Figure 130: Stephan Ulamec,
Philae Lander manager, describes how Philae first landed less than 100
m from the planned Agilkia site (red square). Without functioning
harpoons and thrusters to fix it to the ground there, it rebounded and
shot a kilometer above the comet. Right now, it’s somewhere in
the blue diamond (image credit: ESA)

• Nov. 12, 2014: The following
statement is from John Grunsfeld, astronaut and associate administrator
for NASA’s Science Mission Directorate in Washington, about the
successful comet landing by the European Space Agency’s Rosetta
spacecraft: 219)

- “We congratulate ESA on
their successful landing on a comet today. This achievement represents
a breakthrough moment in the exploration of our solar system and a
milestone for international cooperation. We are proud to be a part of
this historic day and look forward to receiving valuable data from the
three NASA instruments on board Rosetta that will map the comet’s
nucleus and examine it for signs of water.

- “The data collected by
Rosetta will provide the scientific community, and the world, with a
treasure-trove of data. Small bodies in our solar system like comets
and asteroids help us understand how the solar system formed and
provide opportunities to advance exploration. We look forward to
building on Rosetta's success exploring our solar system through our
studies of near earth asteroids and NASA's upcoming asteroid sample
return mission OSIRIS-REx. It’s a great day for space
exploration."

• Nov. 12, 2014: Philae has landed!
ESA’s Rosetta mission has soft-landed its Philae probe on a
comet, the first time in history that such an extraordinary feat has
been achieved. After a tense wait during the seven-hour descent to the
surface of Comet 67P/Churyumov–Gerasimenko, the signal confirming
the successful touchdown arrived on Earth at 16:03 GMT (17:03 CET). 220)

- The confirmation was relayed via
the Rosetta orbiter to Earth and picked up simultaneously by
ESA’s ground station in Malargüe, Argentina and NASA’s
station in Madrid, Spain. The signal was immediately confirmed at
ESA/ESOC, in Darmstadt, and DLR’s Lander Control Center in
Cologne, both in Germany. The first data from the lander’s
instruments were transmitted to the Philae Science, Operations and
Navigation Center at France’s CNES space agency in Toulouse.

- Touchdown was planned to take
place at a speed of around 1 m/s, with the three-legged landing gear
absorbing the impact to prevent rebound, and an ice screw in each foot
driving into the surface.

- Over the next 2.5 days, the lander
will conduct its primary science mission, assuming that its main
battery remains in good health. An extended science phase using the
rechargeable secondary battery may be possible, assuming the Sun
illumination conditions allow and dust settling on the solar panels
does not prevent it. This extended phase could last until March 2015,
after which conditions inside the lander are expected to be too hot for
it to continue operating.

- Science
highlights from the primary phase will include a full panoramic view of
the landing site, including a section in 3D, high-resolution images of
the surface immediately underneath the lander, on-the-spot analysis of
the composition of the comet’s surface materials, and a drill
that will take samples from a depth of 23 cm and feed them to an
onboard laboratory for analysis.

- The lander will also measure the
electrical and mechanical characteristics of the surface. In addition,
low-frequency radio signals will be beamed between Philae and the
orbiter through the nucleus to probe the internal structure. The
detailed surface measurements that Philae makes at its landing site
will complement and calibrate the extensive remote observations made by
the orbiter covering the whole comet.

- While Philae begins its close-up
study of the comet, Rosetta must maneuver from its post-separation path
back into an orbit around the comet, eventually returning to a 20 km
orbit on December 6, 2014. The plans of the ground-breaking Rosetta
mission are to follow the comet around the Sun for 13 months, watching
as its activity changes and its surface evolves.

- Next year, as the comet grows more
active, Rosetta will need to step further back and fly unbound
‘orbits’, but dipping in briefly with daring flybys, some
of which will bring it within just 8 km of the comet center.

- The comet will reach its closest
distance to the Sun on 13 August 2015 at about 185 million km, roughly
between the orbits of Earth and Mars. Rosetta will follow it throughout
the remainder of 2015, as they head away from the Sun and activity
begins to subside.

• Nov. 12, 2014 [at 10:02 CET
(Central European Time)]: The Philae lander has separated from the
Rosetta orbiter, and is now on its way to becoming the first spacecraft
to touch down on a comet. Separation was confirmed at ESA/ESOC at 09:03
GMT. It takes the radio signals from the transmitter on Rosetta 28
minutes and 20 seconds to reach Earth, so separation actually occurred
in space at 08:35 GMT. 221)

- The first signal from Philae is
expected in around two hours, when the lander establishes a
communication link with Rosetta. Philae cannot send its data to Earth
directly – it must do it via Rosetta. Once the link has been
established, the lander will relay via Rosetta a status report of its
health, along with the first science data. This will include images
taken of the orbiter shortly after separation.

- The descent to the surface of
Comet 67P/Churyumov–Gerasimenko will take around seven hours,
during which the lander will take measurements of the environment
around the comet. It will also take images of the final moments of
descent.

- Confirmation of a successful
touchdown is expected in a one-hour window centered on 17:02 GMT. The
first image from the surface is expected some two hours later.

Figure
132: Infographic to summarize the measurements carried out by
Rosetta’s lander, Philae, during its seven-hour descent to Comet
67P/Churyumov–Gerasimenko and immediately after touchdown (image
credit: ESA/ATG medialab)

Figure 133: Rosetta’s
lander Philae took this parting shot of its mothership shortly after
separation. The image was taken with the lander’s CIVA-P imaging
system and captures one of Rosetta's 14 m long solar arrays. It was
stored onboard the lander until the radio link was established with
Rosetta around two hours after separation, and then relayed to Earth
(image credit: ESA, Rosetta, Philae, CIVA). 222)

Figure 135:
First photo released of Comet 67P/C-G taken by the ROLIS camera of
Philae during its descent on Nov. 12 at 14:38:41 UTC from a distance of
~3 km from the surface. The landing site is imaged with a resolution of
about 3 m per pixel (image credit: DLR) 224)

Legend to Figure 135:
The ROLIS camera is a down-looking imager that acquires images during
the descent and doubles as a multispectral close-up camera after the
landing. The aim of the ROLIS experiment is to study the texture and
microstructure of the comet's surface. In the upper right corner a
segment of the Philae landing gear is visible.

• November 11, 2014:
ESA’s comet-chasing Rosetta mission spent much of the second half
of October orbiting Comet 67P/Churyumov–Gerasimenko at less than
10 km from its surface. This selection of previously unpublished
‘beauty shots’, taken by Rosetta’s navigation camera,
presents the varied and dramatic terrain of this mysterious world from
this close orbit phase of the mission. 225)

- Some light contrast enhancements
have been made to emphasize certain features and to bring out features
in the shadowed areas. In reality, the comet is extremely dark —
blacker than coal. The images, taken in black-and-white, are
grey-scaled according to the relative brightness of the features
observed, which depends on local illumination conditions, surface
characteristics and composition of the given area. Some slight
vignetting can also be seen in the corners of some images.

- Only the first image of the 10 scenes presented is displayed in Figure 136. The interested reader may consult reference 225) for a complete display of the imagery.

Figure 136:
This NAVCAM image showcases one of the many pits seen on the surface of
67P/Churyumov–Gerasimenko (image credit: ESA,Rosetta, NAVCAM)

Legend to Figure 136:
Pits like these are thought to be where gas vents into space from the
porous subsurface, carrying with it dusty grains of comet material.
Scientists are keen to learn the role of this pit – and others
– in the development of the comet’s activity, as it gets
ever closer to the Sun.

This single-frame NAVCAM image
measures 1024 x 1024 pixels. It was captured from a distance of 9.9 km
from the center of the comet (about 7.7 km from the surface) at 02:22
GMT on 15 October 2014. At this distance, the image resolution is 84.6
cm/pixel and the size of the image is 866 x 866 m.

• November 4, 2014: The site
where Rosetta’s Philae lander is scheduled to touch down on Comet
67P/Churyumov–Gerasimenko on 12 November now has a name: Agilkia. 226)227)228)229)

Legend to Figure 137:
This image of comet 67P/Churyumov-Gerasimenko was obtained on October
30, 2014 by the OSIRIS scientific imaging system on the Rosetta
spacecraft. The right half is obscured by darkness. The image was taken
from a distance of approximately 30 km.

- The landing site, previously known
as ‘Site J’, is named for Agilkia Island, an island on the
Nile River in the south of Egypt. A complex of Ancient Egyptian
buildings, including the famous Temple of Isis, was moved to Agilkia
from the island of Philae when the latter was flooded during the
building of the Aswan dams last century.

- The name was selected by a jury
comprising members of the Philae Lander Steering Committee as part of a
public competition run 16–22 October by ESA and the German,
French and Italian space agencies.

- Agilkia was one of the most
popular entries – it was proposed by over 150 participants. The
committee selected Alexandre Brouste from France as the overall winner.
As a prize, Mr. Brouste will be invited to ESA’s Space Operations
Control Centre in Darmstadt, Germany, to follow the landing live.

- Although perhaps not quite as
complicated as navigating Rosetta and Philae towards the comet, the
task of choosing a name was by no means simple. More than 8000 entries
from 135 countries were received in one week, showing great creativity
and cultural diversity.

- The entries covered a tremendous
range of themes, from abstract concepts to the names of places on
Earth. As with the winning entry, many suggestions echoed the Egyptian
origins of Rosetta and Philae, named in recognition of milestones in
decoding hieroglyphics, the sacred writing system of ancient Egypt.

Figure 138: The Philae landing site Agilkia is located on the 'head' of Comet 67P/Churyumov-Gerasimenko (image credit: ESA, DLR)

• October 20-24, 2014: In the
“Our week through the lens” series, ESA released an image
of Comet 67P/Churyumov-Gerasimenko (Figure 139), showing jets of cometary activity along almost the entire body of the comet. 230)231)

•
October 15, 2014: ESA has given the green light for its Rosetta mission
to deliver its lander, Philae, to the primary site on
67P/Churyumov–Gerasimenko on 12 November, in the first-ever
attempt at a soft touchdown on a comet. 232)

- Philae’s landing site,
currently known as Site J and located on the smaller of the
comet’s two ‘lobes’, was confirmed on 14 October
following a comprehensive readiness review. — Since the arrival,
the mission has been conducting an unprecedented survey and scientific
analysis of the comet, a remnant of the early phases of the Solar
System’s 4.6 billion-year history.

- At the same time, Rosetta has been
moving closer to the comet: starting at 100 km on 6 August, it is now
just 10 km from the center of the 4 km-wide body. This allowed a more
detailed look at the primary and backup landing sites in order to
complete a hazard assessment, including a detailed boulder census.

- The decision that the mission is
‘Go’ for Site J also confirms the timeline of events
leading up to the landing. Rosetta will release Philae at 08:35
GMT/09:35 CET on 12 November at a distance of approximately 22.5 km
from the center of the comet. Landing will be about seven hours later
at around 15:30 GMT/16:30 CET (Central European Time).

- With a one-way signal travel time
between Rosetta and Earth on 12 November of 28 minutes 20 seconds, that
means that confirmation of separation will arrive on Earth ground
stations at 09:03 GMT/10:03 CET and of touchdown at around 16:00
GMT/17:00 CET.

•
October 10, 2014: The OSIRIS camera on board Europe’s Rosetta
spacecraft has caught a spectacular glimpse of one of the many boulders
that cover the surface of comet 67P/Churyumov-Gerasimenko. With a
maximum extension of 45 m, it is one of the larger structures of this
kind on the comet and stands out among a group of boulders located on
the lower side of 67P’s larger lobe. Since this cluster of
boulders reminded the scientists of the pyramids of Giza, the boulder
has been named Cheops after the largest pyramid within the Giza
Necropolis of Egypt. The boulder-like structures that Rosetta has
revealed on the surface of 67P in the past months are one of the
comet’s most striking and mysterious features. 233)

The large boulder now dubbed Cheops
was seen for the first time in images obtained in early August upon
Rosetta’s arrival at the comet (Figure 152).
In the past weeks as Rosetta has navigated closer and closer to the
comet’s surface, OSIRIS imaged the unique structure again –
but this time with a much higher resolution of 50 cm/pixel. The image
of Figure 141 was acquired on Sept. 19, 2014 from a distance of 28.5 km. 234)

Figure 141:
Close-up of the boulder Cheops as it casts a long shadow on the surface
of comet 67P/Churyumov-Gerasimenko, (image credit: ESA, Rosetta,MPS for
OSIRIS Team MPS, UPD, LAM, IAA, SSO, INTA, UPM, DASP, IDA)

Legend to Figure 141: The boulder is the largest one of the group of boulders in the center of image 152.

• On October 8, 2014, ESA
reports of grooves found on asteroid Lutetia when Rosetta flew past
Lutetia at a distance of 3168 km in July 2010. The spacecraft took
images of the 100 km-wide asteroid for about two hours during the
flyby, revealing numerous impact craters and hundreds of grooves all
over the surface. 235)

Impact
craters are commonly seen on all Solar System worlds with solid
surfaces, recording an intense history of collisions between bodies.
However, grooves are much less prevalent. To date, they have been
discovered by visiting spacecraft only on the martian moon Phobos and
the asteroids Eros and Vesta.

Figure 142: Tracing Lutetia's grooves,”(image credit: ESA)

• Oct. 3, 2014: Comet
67P/Churyumov-Gerasimenko's dimensions, as measured from images taken
by Rosetta's OSIRIS imaging system. The images shown in the graphic
were taken by Rosetta's navigation camera on August 19, 2014. - The
larger lobe of the comet measures 4.1 x 3.2 x 1.3 km, while the smaller
lobe is 2.5 x 2.5 x 2.0 km. 236)

- One of the key things is the
so-called “shape model”, meaning a 3D model of the comet
based on images from the OSIRIS and NAVCAM cameras. Because roughly 30%
of the ‘dark side’ of 67P/C-G has not been resolved and
analyzed fully yet, the shape model is very incomplete over those
regions. As a result, some of the derived parameters for the comet are
only best estimates at present. These include the volume and the global
density, the latter depending on the mass and the volume. 237)

• September 26, 2014: The
Rosetta mission will deploy its lander, Philae, to the surface of Comet
67P/Churyumov-Gerasimenko on November 12, 2014. Philae's landing site,
currently known as Site J, is located on the smaller of the comet's two
'lobes', with a backup site on the larger lobe. The sites were selected
just six weeks after Rosetta arrived at the comet on 6 August,
following its 10-year journey through the Solar System. 238)

- The primary landing site was
chosen from five candidates during the Landing Site Selection Group
meeting held on 13–14 September 2014.

Figure 144: Rosetta's NAVCAM
camera took this image of Comet 67P/Churyumov-Gerasimenko on 21
September, from a distance of 27.8 km from the comet center. The image
covers an area of about 2 km x 1.9 km and focuses on the smaller of the
two comet lobes. The primary landing site J is 'above' the distinctive
depression in this view (image credit: ESA, Rosetta). 239)

Figure 145:
Site J is located on the head of Comet 67P/Churyumov–Gerasimenko.
An inset showing a close up of the landing site is also shown (image
credit: ESA, Rosetta, MPS for OSIRIS Team MPS, UPD, LAM, IAA SSO, INTA,
UPM, DASP, IDA)

Legend to Figure 145:
The inset image was taken by Rosetta's OSIRIS narrow-angle camera on 20
August 2014 from a distance of about 67 km. The image scale is 1.2
m/pixel. The background image was taken on 16 August from a distance of
about 100 km. The comet nucleus is about 4 km across.

•
Sept. 5, 2014: After examining the images of the comet obtained on
August 6, members of the Rosetta team at ESA couldn’t help but
notice that Comet 67P/C-G appeared to be a very oddly shaped object.
Its peculiar shape led them to nickname the comet the “rubber
duck”. As the team continues to study Comet 67P/C-G, the
scientists are looking to get a better understanding of its surface
properties. 240)

- Orbit: Rosetta is in an orbit
about the comet at an avearge distance of ~30 km. The team is planning
to occasionally lower the spacecraft’s orbit to about 10 km above
the comet’s surface or possibly even lower when the
Rosetta’s attached Philae lander is deployed in November.

- An ideal landing site for Philae is that would be about 1 km2
in size and able to provide enough sunlight to charge the probe’s
battery. - Since the comet’s gravity is so low, the probe will
most likely bounce when it first touches down, so ESA engineers have
equipped it with two harpoons and some ice screws to keep the probe
steady and attached to 67P/C-G’s surface.

• August 25, 2014: Using
detailed information collected by ESA’s Rosetta spacecraft during
its first two weeks at Comet 67P/Churyumov-Gerasimenko, five locations
have been identified as candidate sites to set down the Philae lander
in November – the first time a landing on a comet has ever been
attempted. 241)

The approximate locations of the five regions are marked on these OSIRIS narrow-angle camera images of Figure 146 taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.

The sites were assigned a letter
from an original pre-selection of 10 possible sites identified A
through J. The lettering scheme does not signify any ranking. Three
sites (B, I and J) are located on the smaller of the two lobes of the
comet and two sites (A and C) are located on the larger lobe.

The landing of Philae is expected
to take place in mid-November when the comet is about 450 million km
from the Sun, before activity on the comet reaches levels that might
jeopardise the safe and accurate deployment of Philae to the
comet’s surface, and before surface material is modified by this
activity.

The comet is on a 6.5 year orbit
around the Sun and today is 522 million km from it. At their closest
approach on 13 August 2015, just under a year from now, the comet and
Rosetta will be 185 million km from the Sun, meaning an eightfold
increase in the light received from the Sun.

While Rosetta and its scientific
instruments will watch how the comet evolves as heating by the Sun
increases, observing how its coma develops and how the surface changes
over time, the lander Philae and its instruments will be tasked with
making complementary in situ measurements at the comet’s surface.
The lander and orbiter will also work together using the CONSERT
experiment to send and detect radio waves through the comet’s
interior, in order to characterise its internal structure.

• Comet characterization (Ref. 246):
This phase was mainly driven by the technically challenging and time
critical need for the operations team to develop engineering models of
the comet such that the proper orbit phase and the landing site
selection process could start.

For the team to design and plan the
next phase it was necessary to have a pretty accurate model of the
gravity field and of the comet attitude with an associated reference
frame. In total absence of further information about the comet, the
first step of the process was to catalog the so-called landmarks i.e.
evident features of the comet’s surface that could easily be
recognized in the images and used as navigation references. The
position of these landmarks in subsequent images and the traditional
radiometric data (ranging and Doppler) were then fed into the orbit
determination system which essentially consisted of an estimator of:

- Spacecraft position and velocity

- Comet position and velocity

- Comet spin axis

- Comet attitude evolution (thus rotation period)

- Comet gravity potential (thus mass) and position of the center of mass

- Comet shape.

Due to the extremely large
inaccuracy in the a priori knowledge of the comet’s gravity
potential it was not possible to inject the spacecraft onto a proper
captured orbit. Therefore all these measurements were collected from a
so-called “pyramid orbit” consisting of a sequence of three
portions of hyperbolic arcs flown in front of the comet, on the
illuminated side (Figure 147), at a
distance varying between 115 and 90 km. In this way the spacecraft was
not captured by the gravity but would still be in a position to have
its orbit affected by it and have the possibility to observe the comet
from different angles.

Figure 147: Pyramid orbits (image credit: ESA/ESOC)

This triangular trajectory was
flown for 10 days and, after a transfer of 7 days, another triangle was
flown at distances ranging from 70 to 50 km (Figure 148).

The timeline of events reported in Table 18 shows the time pressure the operations team was encountering during this phase (major events in bold italics).
This was mainly due to the need of starting the landing site selection
process as soon as possible to meet the delivery deadline of mid
November 2014. During this phase the conceived mission planning concept
revealed to be extremely robust and was key to the success. At the same
time the full complement of scientific instruments was performing
uninterrupted scientific measurements.

The
estimation phase of cometary parameters and the development of the
relevant models was conducted according to plan with the first full
operational set released on 14 August at the end of the 2nd segment of
the first triangle. At this stage also the shape model of the comet was
available with an accuracy level beyond the expectations. The only
estimation process that took longer to converge was the one determining
the position of the center of mass of the comet. The comet was found
– unexpectedly - not to be nutating at all and this had the
consequence that the position of the center of mass could only be
estimated from the spacecraft orbital reconstruction without any aid
from the optical images.

This phase also marked the
transition from far approach optical navigation, where images were used
to compute the position of the comet with respect to fixed stars, to
proximity navigation, where the spacecraft position with respect to the
comet is determined by the position of the landmarks in the images. Due
to the active environment around the comet, where rather unpredictable
aerodynamic forces act on the spacecraft, the proximity optical
navigation method will constitute the basis for navigation throughout
the mission. This imposes specific constraints for operations planning
(e.g. regular image taking sessions capable of coping with pointing
uncertainties) and execution (e.g. availability of recent optical
navigation data for orbit determination and commands generation
sessions) that have to be considered by the operations team.

The models developed at this stage were accurate enough to kick-off the next two steps of the mission:

The whole orbiting plan for the
phase immediately following the comet characterization (i.e. the 2
pyramid orbits) was fully dependant on the actual comet parameters,
mainly spin axis and mass. With these parameters resolved, it was
possible for the operations team to plan and release the operational
orbits. These include an initial orbital phase at ca. 29 km radius with
orbital plane tilted 60º away from the Sun direction then followed
by a phase where the orbit radius is reduced to ca. 19 km and the
orbital plane is moved to the terminator plane (Figure 149).

A further reduction of the orbit
radius will be conducted only if allowed by the dynamical stability of
the comet environment. This strategy was necessary to combine the wish
of the scientists to have good observation conditions with the need to
minimize the spacecraft surface exposed to the – mostly radial -
gas flow coming from the comet. The terminator plane, being
perpendicular to the Sun direction, ensures that only the edge of the
solar arrays is exposed to the gas flow, thus minimizing the
aerodynamic forces acting on the spacecraft.

This orbital phase is considered as
most valuable by the vast majority of the scientists in charge of
Rosetta’s on-board instruments because it might be the only
opportunity to fly orbits so close to the surface, at least in this
initial comet operations phase. In any case, with the decrease of the
heliocentric distance the comet is expected to increase its activity
and will force the operations team to fly at larger distances from the
comet nucleus.

• On
6 August 2014, the Rosetta mission achieved a significant milestone by
becoming the first mission to rendezvous with a comet. During the
coming months, Rosetta will orbit the comet, deploy the Philae lander
(in November, 2014), and accompany the comet through perihelion (August
2015) until the nominal end of the mission. 242)

- After a decade-long journey
chasing its target, ESA’s Rosetta has today become the first
spacecraft to rendezvous with a comet, opening a new chapter in Solar
System exploration. 243)

Comet
67P/Churyumov–Gerasimenko and Rosetta now lie 405 million km from
Earth, about half way between the orbits of Jupiter and Mars, rushing
towards the inner Solar System at nearly 55 000 km/hr.

The comet is in an elliptical 6.5
year orbit that takes it from beyond Jupiter at its furthest point, to
between the orbits of Mars and Earth at its closest to the Sun. Rosetta
will accompany it for over a year as they swing around the Sun and back
out towards Jupiter again.

- The comet
began to reveal its personality while Rosetta was on its approach.
Images taken by the OSIRIS camera between late April and early June
showed that its activity was variable. The comet’s
‘coma’ – an extended envelope of gas and dust –
became rapidly brighter and then died down again over the course of
those six weeks.

In the same period, first
measurements from the MIRO (Microwave Instrument) on the Rosetta
Orbiter suggested that the comet was emitting water vapor into space at
about 300 ml/s (milliliter).

Meanwhile, VIRTIS (Visible and
Infrared Thermal Imaging Spectrometer) measured the comet’s
average temperature to be about –70ºC, indicating that the
surface is predominantly dark and dusty rather than clean and icy.

Then, stunning images taken from a
distance of about 12 000 km began to reveal that the nucleus comprises
two distinct segments joined by a ‘neck’, giving it a
duck-like appearance. Subsequent images showed more and more detail
(Figures 150 and 151) — the most recent, highest-resolution image was downloaded from the spacecraft on August 8 (Figure 152).

Legend to Figure 152:
Stunning close up detail focusing on a smooth region on the
‘base’ of the ‘body’ section of comet
67P/Churyumov-Gerasimenko. The image was taken by Rosetta’s
OSIRIS narrow-angle camera and downloaded today, 6 August. The image
clearly shows a range of features, including boulders, craters and
steep cliffs. - The image was taken from a distance of 130 km and the
image resolution is 2.4 m.

• August 2, 2014: An image of the comet's activity was acquired (Figure 153)
with the OSIRIS camera from a distance of 550 km. The exposure time of
the image was 330 seconds and the comet nucleus is saturated to bring
out the detail of the comet activity.

• During May - August 2014,
Rosetta was executing a series of 10 OCMs (Orbit Correction Maneuvers)
to line itself up for arrival at comet 67P on August 6 –
approximately one burn every two weeks in May and June, and one per
week in July (see also Table 19). 245)

- The first, on 7 May, was quite small, achieving a ΔV of just 20 m/s (with respect to the comet).

- The second
was carried out on 21 May, referred to as Big Burn, it lasted for 7 hrs
and 16minutes, used 218 kg of propellant and delivered a total ΔV
of 289.59 m/s. During the OCM, teams at ESOC monitored parameters such
as temperature and pressure of the four thrusters.

- Big Burn2 was on June 4 resulting
in a ΔV of 269.5 m/s. At this event, Rosetta was 425, 250 km from
the comet, approaching at a relative speed of 463 m/s. The one-way
radio signal time to Earth was 25 min:56 seconds.

• Following the January 2014
wake-up of the Rosetta spacecraft after the hibernation period, Rosetta
conducted the delicate approach phase during which it slowly discovered
its unexpected irregular shape. In order to complete the rendezvous as
planned Rosetta had to reduce the miss- distance to a few tens of km
and the relative velocity down to ca. 1 m/s. This could have been
possible by imparting to the spacecraft a n acceleration of ca. 775 m/s
at the right time on June 3, resulting in the two objects flying on the
same heliocentric orbit. - However, the uncertainties in the knowledge
of the orbit of the comet (ca. 10000 km on the position) were such that
it was not possible to conduct this operation without further
information. For this reason an optical navigation campaign was
designed and conducted such that by means of images taken by the
spacecraft the flight controllers at the Mission Control Center at ESOC
were in a position to reconstruct the relative trajectories of the two
bodies with incremental accuracy. Figure 154
shows how Rosetta was flying on the illuminated side of the comet, a
mandatory configuration for the conduct of the optical navigation
campaign. 246)

The rendezvous maneuver was split
into a sequence of 10 single maneuvers spread over a period of ca. 3
months, during which the parameters of the relative trajectory were
resolved to the accuracy required for a successful orbit insertion.
Table 19 reports the details of the maneuvers as conducted. The last four columns report respectively:

- the date of the comet flyby in case the manoeuvre would not have been executed before the relevant flyby date

- the distance of the flyby as determined with the orbit reconstruction process

- the 3σ uncertainty associated to this distance

- the time margin the operations
team had to complete the manoeuvre before the spacecraft would fly-past
the comet and find itself to have to come back on the night side of the
comet with limited optical navigation capabilities.

Maneuver data

Flyby data in case of missed maneuver

Date

ΔV (m/s)

Distance to comet (km)

Date

Mission distance (km)

3σ uncertainty (km)

Margin (days)

07.05.14

20.0

1,921,331

03.06.14

50,304

100

27

21.05.14

289.6

1,007,849

05.06.14

47,510

100

15

04.06.14

88.7

428,208

14.06.14

27,298

20

10

18.06.14

88.7

178,913

28.06.14

13,804

14

10

02.07.14

58.7

50,154

07.07.14

10,093

5

5

09.07.14

25.7

21,300

14.07.14

4,912

3

5

16.07.14

10.9

9,134

21.07.14

2,415

4

5

23.07.14

4.8

3,920

28.07.14

1,010

3

5

03.08.14

3.2

534

04.08.14

219

2

1

06.08.14

0.9

120

07.08.14

93

3

1

Table 19: Sequence of rendezvous maneuvers

The optical navigation campaign was
conducted with images initially taken by the OSIRIS/NAC and later by
the spacecraft Navigation Camera (NAVCAM). The very first set of images
acquired with the NAC ( Figure 155) at
the end of March 2014, when the comet was at ca. 4.8 million km from
the spacecraft, allowed the operations team to determine that the comet
was ca. 2000 km away from its expected position i.e. well within the
expected uncertainty.

• After the successful wakeup,
Rosetta and Philae underwent a post hibernation commissioning. The
orbiter instruments (like e.g. the OSIRIS cameras, VIRTIS, MIRO, Alice
and ROSINA) characterize the target comet and its environment to allow
landing site selection and the definition of a separation, descent and
landing (SDL) strategy for the Lander (Ref. 63).

- The first switch-on of the Philae lander
took place on March 28, 2014, when an updated software for the CDMS
(Central Data Management System) was uploaded. This activity was
followed by three commissioning blocks, where all lander subsystems and
instruments were activated, EEPROMs have been refreshed and in some
cases new software was uploaded. No major degradation has been
observed, the lander was found to be in a state very similar as during
the checkouts before entering hibernation.

- The lander was switched on for
several further occasions, before the actual SDL (Separation, Descent
and Landing) sequence will be initiated in November. The so-colled PDCS
(Pre-Delivery Calibration and Science) phase includes background
measurements and “sniffing” of the mass spectrometers
(PTOLEMY and COSAC), calibration of the CIVA cameras as well as imaging
of the comet nucleus, parallel operations of ROMAP with RPC (magnetic
field, ion environment) and activation of CONSERT. The solar generator
performance has been verified and the secondary batteries are cycled
for capacity degradation measurement.

• January 20, 2014 : ESA's Rosetta orbiter woke up from a 31 month hibernation period
at 18:18 GMT to begin an ambitious year of operations to become the
first craft to rendezvous with a comet, follow it as it makes its close
approach to the Sun and deploy the Philae lander onto its surface. 247)

- Operating on solar energy alone,
Rosetta was placed into a deep space slumber in June 2011 as it cruised
out to a distance of nearly 800 million km from the warmth of the Sun,
beyond the orbit of Jupiter. Now, as Rosetta’s orbit has brought
it back to within ‘only’ 673 million km from the Sun, there
is enough solar energy to power the spacecraft fully again. - Thus
today, still about 9 million km from the comet, Rosetta’s
pre-programmed internal ‘alarm clock’ woke up the
spacecraft. After warming up its key navigation instruments, coming out
of a stabilising spin, and aiming its main radio antenna at Earth,
Rosetta sent a signal to let mission operators know it had survived the
most distant part of its journey.

- The signal
was received by both of NASA’s Goldstone and Canberra ground
stations during the first window of opportunity the spacecraft had to
communicate with Earth. It was immediately confirmed at ESOC in
Darmstadt and the successful wake-up announced.

• June 8, 2011: The final
command placing ESA's Rosetta comet-chaser into deep-space hibernation
was sent earlier today. With virtually all systems shut down, the probe
will now coast for 31 months until waking up in 2014 for arrival at its
comet destination. 248)

The event marks the end of the
hugely successful first phase of Rosetta's ten-year cruise and the
start of a long, dark hibernation during which all instruments and
almost all control systems will be silent. The deep sleep is made
necessary by the craft's enormous distance from the Sun and the
weakness of the sunlight falling on its solar panels, which cannot
produce enough electricity to power the probe fully.

Only the computer and several
heaters will remain active. These will be automatically controlled to
ensure that the entire satellite doesn't freeze as its orbit takes it
from 660 million km from the Sun out to 790 million km and back between
now and 2014.

• July 10, 2010: Asteroid
Lutetia has been revealed as a battered world of many craters.
ESA’s Rosetta mission has returned the first close-up images of
the asteroid showing it is most probably a primitive survivor from the
violent birth of the Solar System. The flyby was a spectacular success
with Rosetta performing faultlessly. Closest approach took place at
18:10 CEST (Central European Standard Time), at a distance of 3162 km. 249)

- Lutetia is one of the largest
objects orbiting within the main asteroid belt between Mars and
Jupiter. Rosetta's encounter revealed an intriguing object which has
survived since the birth of the planets, some 4.5 billion years ago. 250)

Discovered in 1852, Lutetia was
among the first objects to be classified as an M-type (metallic)
asteroid, but radar observations revealed an unusually low albedo, or
reflectivity, that was inconsistent with a metallic surface. Meanwhile,
spectra obtained at visible and infrared wavelengths found similarities
with meteorites known as enstatite chondrites and with carbonaceous
chondrite meteorites, typically associated with C-type asteroids.

• Sept. 5, 2008: The Rosetta
control room at ESA/ESOC received the first radio signal after closest
approach to asteroid (2867) Steins at 22:14 CEST, confirming a smooth
fly-by. The closest approach was at a distance of 800 km.
Rosetta’s relative speed with respect to asteroid Steins was 8.6
km/s, or about 31 000 km/h. 251)

- The first images from
Rosetta’s OSIRIS imaging system and VIRTIS infrared spectrometer
were derived from raw data this morning and have delivered spectacular
results. 252)

- The observations by OSIRIS and
VIRTIS on Rosetta brought new information that could not have been
gained from the ground. The dimensions of Steins were found to be 6.67
x 5.81 x 4.47 km3. Rosetta scientists believe that Steins
was part of a larger differentiated object that had broken up. It was
later struck by other objects, creating impact craters. However, the
interior is thought to be a rubble pile and the asteroid will
eventually break up. 253)

Figure 158: Artist's view of the Rosetta spacecraft on its way to Comet 67P/Churyumov–Gerasimenko (image credit: DLR) 254)

• On Feb. 25, 2007, the Rosetta spacecraft encountered planet Mars for a gravity assit. The image of Figure 159
was taken just four minutes before the spacecraft reached closest
approach, about 1,000 km from the planet’s surface. An area close
to the Syrtis region is visible on the planet’s disk (Ref. 58).

• After launch on March 2,
2004, the Rosetta spacecraft was first inserted into a parking orbit,
before being sent on its way towards the outer Solar System. 255)

Unfortunately, no existing rocket,
not even the powerful European-built Ariane-5, has the capability to
send such a large spacecraft directly to Comet
67P/Churyumov-Gerasimenko. Instead, Rosetta will bounce around the
inner Solar System like a ‘cosmic billiard ball’, circling
the Sun almost four times during its ten-year trek to Comet
67P/Churyumov-Gerasimenko.

Along this
roundabout route, Rosetta will enter the asteroid belt twice and gain
velocity from gravitational ‘kicks’ provided by close
flybys of Mars (2007) and Earth (2005, 2007 and 2009).

• Figure 160
is an image of the Rosetta mission destination in 2014, namely the
Comet 67P/Churyumov–Gerasimenko, acquired by Hubble almost a year
before to the launch of the Rosetta mission. 256)

Results from NASA's Hubble Space
Telescope played a major role in preparing ESA's ambitious Rosetta
mission for its new target, comet 67P/Churyumov-Gerasimenko (67P/C-G).
For the first time in history, Rosetta will land a probe on a comet and
study its origin. Hubble precisely measured the size, shape, and
rotational period of comet 67P/C-G.

Hubble's
observations revealed that comet 67P/C-G is approximately a
three-by-two mile, football-shaped object on which it is possible to
land. Mission scientists were concerned that the solid nucleus could be
nearly 3.6 miles (6 km) across. The higher gravity on a comet that size
might make a soft landing more difficult.

95)
The discovery of xenon by Rosetta at Comet 67P/Churyumov-Gerasimenko
was announced during a Royal Society meeting in London, UK, and on the
ESA Rosetta blog in June 2016, shortly after the scientists had made
the detection. This is the first peer-reviewed study based on those
measurements.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).